Check outstanding RPC is not null

Before it receives the handling of the command.
The outstanding RPC can be null in case of async call.

Author: acogoluegnes

pom.xml

@@ -196,7 +196,7 @@
 
               Currently, those test brokers will listen to the default
               TCP ports of 5672 and 5673 for AMQP and 5671 and 5670
-              for AMQP+TLS. Unforunately, this means no other standard
+              for AMQP+TLS. Unfortunately, this means no other standard
               RabbitMQ can run on the same host at the same time.
               -->
               <execution>

src/main/java/com/rabbitmq/client/impl/AMQChannel.java

@@ -143,8 +143,12 @@ public void handleCompleteInboundCommand(AMQCommand command) throws IOException
         if (!processAsync(command)) {
             // The filter decided not to handle/consume the command,
             // so it must be some reply to an earlier RPC.
-            nextOutstandingRpc().handleCommand(command);
-            markRpcFinished();
+            RpcContinuation nextOutstandingRpc = nextOutstandingRpc();
+            // the outstanding RPC can be null when calling Channel#asyncRpc
+            if(nextOutstandingRpc != null) {
+                nextOutstandingRpc.handleCommand(command);
+                markRpcFinished();
+            }
         }
     }