Check that splitIndex is not negative

Author: rkalis

packages/cashc/src/semantic/TypeCheckTraversal.ts

@@ -406,6 +406,10 @@ function inferTupleType(node: BinaryOpNode): Type {
     return new TupleType(new BytesType(splitIndex), new BytesType());
   }
 
+  if (splitIndex < 0) {
+    throw new IndexOutOfBoundsError(node);
+  }
+
   if (splitIndex > expressionType.bound) {
     throw new IndexOutOfBoundsError(node);
   }

packages/cashc/test/compiler/IndexOutOfBoundsError/split_index_negative.cash

@@ -0,0 +1,6 @@
+contract Test(bytes8 b) {
+    function spend() {
+        bytes x = b.split(-4)[0];
+        require(x != b);
+    }
+}