message: [C++] Fixed crash when copying table with empty shared strings
diff: @@ -579,7 +579,7 @@ class FlatBufferBuilder {
  /// @param[in] str A const pointer to a `String` struct to add to the buffer.
  /// @return Returns the offset in the buffer where the string starts
  Offset<String> CreateSharedString(const String *str) {
  - return CreateSharedString(str->c_str(), str->size());
  + return str ? CreateSharedString(str->c_str(), str->size()) : 0;
  }
  /// @cond FLATBUFFERS_INTERNAL
  