[C++] insert null into vector (google#8424)

include/flatbuffers/buffer.h

@@ -116,6 +116,8 @@ struct IndirectHelper<OffsetT<T>> {
     // Offsets are relative to themselves, so first update the pointer to
     // point to the offset location.
     const uint8_t *const offset_location = p + i * element_stride;
+    if (*offset_location == 0)
+        return nullptr;
 
     // Then read the scalar value of the offset (which may be 32 or 64-bits) and
     // then determine the relative location from the offset location.

include/flatbuffers/flatbuffer_builder.h

@@ -317,7 +317,10 @@ template<bool Is64Aware = false> class FlatBufferBuilderImpl {
   template<typename T, template<typename> class OffsetT = Offset>
   uoffset_t PushElement(OffsetT<T> off) {
     // Special case for offsets: see ReferTo below.
-    return PushElement(ReferTo(off.o));
+      if (off.o == 0)
+          return PushElement(0);
+      else
+          return PushElement(ReferTo(off.o));;
   }
 
   // When writing fields, we track where they are, so we can create correct
@@ -381,12 +384,12 @@ template<bool Is64Aware = false> class FlatBufferBuilderImpl {
   }
 
   template<typename T, typename T2> T ReferTo(const T off, const T2 size) {
-    FLATBUFFERS_ASSERT(off && off <= size);
+    FLATBUFFERS_ASSERT(off <= size);
     return size - off + static_cast<T>(sizeof(T));
   }
 
   template<typename T> T ReferTo(const T off, const T size) {
-    FLATBUFFERS_ASSERT(off && off <= size);
+    FLATBUFFERS_ASSERT(off <= size);
     return size - off + static_cast<T>(sizeof(T));
   }