Add assertion when building MapBuffer

Summary:
This diff adds an assertion when trying to build a MapBuffer with invalid data

changelog: [internal] internal

Reviewed By: sammy-SC

Differential Revision: D27864309

fbshipit-source-id: 6601388e56be18ded0675f92cce009a577828c16

ReactCommon/react/renderer/mapbuffer/MapBufferBuilder.cpp

@@ -160,6 +160,10 @@ void MapBufferBuilder::putMapBuffer(Key key, MapBuffer &map) {
 }
 
 MapBuffer MapBufferBuilder::build() {
+  react_native_assert(
+      (keyValues_ != nullptr) &&
+      "Error when building mapbuffer with invalid datastructures.");
+
   // Create buffer: [header] + [key, values] + [dynamic data]
   int bufferSize = keyValuesOffset_ + dynamicDataOffset_;