Fix Reset in FlatbufferFixedAllocatorArray

The object wasn't usable after Reset...

Change-Id: I996778c36d338b9662c039f552f6f803fe0af169
diff --git a/aos/flatbuffers.h b/aos/flatbuffers.h
index 5da7466..2d0c92b 100644
--- a/aos/flatbuffers.h
+++ b/aos/flatbuffers.h
@@ -264,9 +264,12 @@
   }
 
   void Reset() {
-    CHECK(!allocator_.is_allocated()) << ": May not reset while building";
+    CHECK(!allocator_.is_allocated() || data_ != nullptr)
+        << ": May not reset while building";
     fbb_ = flatbuffers::FlatBufferBuilder(Size, &allocator_);
     fbb_.ForceDefaults(true);
+    data_ = nullptr;
+    size_ = 0;
   }
 
   flatbuffers::FlatBufferBuilder *fbb() {