Fixed CreateStruct not testing for nesting.

Change-Id: I16bf46682ad68c62b778ed1514b6b706c8cb8f6f
Tested: on Linux.

include/flatbuffers/flatbuffers.h

@@ -1446,6 +1446,7 @@ class FlatBufferBuilder
 
   /// @brief Write a struct by itself, typically to be part of a union.
   template<typename T> Offset<const T *> CreateStruct(const T &structobj) {
+    NotNested();
     Align(AlignOf<T>());
     buf_.push_small(structobj);
     return Offset<const T *>(GetSize());