Ensure buffer is large enough to contain message.

Previously, when generating arbitrary encoded values the generator would
not necessarily extend the buffer to the size necessary to hold the
message, e.g., if optional fields were left unset at the end of the
message block. We now call `checkLimit` to "reserve" space for the full
block length at the message and group level.

This fixes an exception seen in the slow tests:

```
java.lang.IndexOutOfBoundsException: index=0 length=129 capacity=128
	at org.agrona.AbstractMutableDirectBuffer.boundsCheck0(AbstractMutableDirectBuffer.java:1719)
	at org.agrona.AbstractMutableDirectBuffer.getBytes(AbstractMutableDirectBuffer.java:464)
	at org.agrona.io.DirectBufferInputStream.read(DirectBufferInputStream.java:175)
	at uk.co.real_logic.sbe.properties.DtosPropertyTest.writeInputFile(DtosPropertyTest.java:199)
	at uk.co.real_logic.sbe.properties.DtosPropertyTest.cppDtoEncodeShouldBeTheInverseOfDtoDecode(DtosPropertyTest.java:147)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at net.jqwik.engine.execution.CheckedPropertyFactory.lambda$createRawFunction$1(CheckedPropertyFactory.java:84)
	at net.jqwik.engine.execution.CheckedPropertyFactory.lambda$createRawFunction$2(CheckedPropertyFactory.java:91)
	at net.jqwik.engine.properties.CheckedFunction.execute(CheckedFunction.java:17)
	at net.jqwik.api.lifecycle.AroundTryHook.lambda$static$0(AroundTryHook.java:57)
	at net.jqwik.engine.execution.lifecycle.HookSupport.lambda$wrap$2(HookSupport.java:48)
	at net.jqwik.engine.hooks.lifecycle.TryLifecycleMethodsHook.aroundTry(TryLifecycleMethodsHook.java:57)
	at net.jqwik.engine.execution.lifecycle.HookSupport.lambda$wrap$3(HookSupport.java:53)
	at net.jqwik.engine.execution.lifecycle.HookSupport.lambda$wrap$2(HookSupport.java:48)
	at net.jqwik.engine.hooks.lifecycle.BeforeTryMembersHook.aroundTry(BeforeTryMembersHook.java:69)
	at net.jqwik.engine.execution.lifecycle.HookSupport.lambda$wrap$3(HookSupport.java:53)
	at net.jqwik.engine.execution.CheckedPropertyFactory.lambda$createTryExecutor$0(CheckedPropertyFactory.java:60)
	at net.jqwik.engine.execution.lifecycle.AroundTryLifecycle.execute(AroundTryLifecycle.java:23)
	at net.jqwik.engine.properties.GenericProperty.testPredicate(GenericProperty.java:166)
	at net.jqwik.engine.properties.GenericProperty.check(GenericProperty.java:68)
	at net.jqwik.engine.execution.CheckedProperty.check(CheckedProperty.java:67)
	at net.jqwik.engine.execution.PropertyMethodExecutor.executeProperty(PropertyMethodExecutor.java:90)
	at net.jqwik.engine.execution.PropertyMethodExecutor.executeMethod(PropertyMethodExecutor.java:69)
	at net.jqwik.engine.execution.PropertyMethodExecutor.lambda$execute$0(PropertyMethodExecutor.java:49)
	at net.jqwik.api.lifecycle.AroundPropertyHook.lambda$static$0(AroundPropertyHook.java:46)
	at net.jqwik.engine.execution.lifecycle.HookSupport.lambda$wrap$0(HookSupport.java:26)
	at net.jqwik.api.lifecycle.PropertyExecutor.executeAndFinally(PropertyExecutor.java:39)
	at net.jqwik.engine.hooks.lifecycle.PropertyLifecycleMethodsHook.aroundProperty(PropertyLifecycleMethodsHook.java:56)
	at net.jqwik.engine.execution.lifecycle.HookSupport.lambda$wrap$1(HookSupport.java:31)
	at net.jqwik.engine.execution.lifecycle.HookSupport.lambda$wrap$0(HookSupport.java:26)
	at net.jqwik.engine.hooks.statistics.StatisticsHook.aroundProperty(StatisticsHook.java:37)
	at net.jqwik.engine.execution.lifecycle.HookSupport.lambda$wrap$1(HookSupport.java:31)
	at net.jqwik.engine.execution.lifecycle.HookSupport.lambda$wrap$0(HookSupport.java:26)
	at net.jqwik.engine.hooks.lifecycle.AutoCloseableHook.aroundProperty(AutoCloseableHook.java:13)
	at net.jqwik.engine.execution.lifecycle.HookSupport.lambda$wrap$1(HookSupport.java:31)
	at net.jqwik.engine.execution.PropertyMethodExecutor.execute(PropertyMethodExecutor.java:47)
	at net.jqwik.engine.execution.PropertyTaskCreator.executeTestMethod(PropertyTaskCreator.java:166)
	at net.jqwik.engine.execution.PropertyTaskCreator.lambda$createTask$1(PropertyTaskCreator.java:51)
	at net.jqwik.engine.execution.lifecycle.CurrentDomainContext.runWithContext(CurrentDomainContext.java:28)
	at net.jqwik.engine.execution.PropertyTaskCreator.lambda$createTask$2(PropertyTaskCreator.java:50)
	at net.jqwik.engine.execution.pipeline.ExecutionTask$1.lambda$execute$0(ExecutionTask.java:31)
	at net.jqwik.engine.execution.lifecycle.CurrentTestDescriptor.runWithDescriptor(CurrentTestDescriptor.java:17)
	at net.jqwik.engine.execution.pipeline.ExecutionTask$1.execute(ExecutionTask.java:31)
	at net.jqwik.engine.execution.pipeline.ExecutionPipeline.runToTermination(ExecutionPipeline.java:82)
	at net.jqwik.engine.execution.JqwikExecutor.execute(JqwikExecutor.java:46)
	at net.jqwik.engine.JqwikTestEngine.executeTests(JqwikTestEngine.java:70)
	at net.jqwik.engine.JqwikTestEngine.execute(JqwikTestEngine.java:53)
```

Author: ZachBray

sbe-tool/src/propertyTest/java/uk/co/real_logic/sbe/properties/arbitraries/SbeArbitraries.java

@@ -801,6 +801,7 @@ private static Arbitrary<Encoder> groupsEncoder(
                 {
                     final int offset = limit.get();
                     fieldsEncoder.encode(builder, buffer, offset, null);
+                    buffer.checkLimit(offset + blockLength);
                     limit.set(offset + blockLength);
                     builder.appendLine().append("limit: ").append(offset).append(" -> ").append(limit.get());
                     groupsEncoder.encode(builder, buffer, NULL_VALUE, limit);
@@ -929,6 +930,7 @@ private static Arbitrary<Encoder> messageValueEncoder(
                 final int headerLength = 8;
                 fields.encode(builder, buffer, offset + headerLength, null);
                 final int oldLimit = limit.get();
+                buffer.checkLimit(offset + headerLength + blockLength);
                 limit.set(offset + headerLength + blockLength);
                 builder.appendLine().append("limit: ").append(oldLimit).append(" -> ").append(limit.get());
                 groups.encode(builder, buffer, NULL_VALUE, limit);