Merge pull request #14719 from alalek:update_protobuf_3.5.2

3rdparty/protobuf/README.md

 Project: Protocol Buffers - Google's data interchange format
-Source code: https://github.com/google/protobuf
-Version: 3.5.1
+Source code: https://github.com/protocolbuffers/protobuf
+Version: 3.5.2

3rdparty/protobuf/src/google/protobuf/stubs/io_win32.cc

@@ -91,7 +91,7 @@ struct CharTraits<wchar_t> {
 
 template <typename char_type>
 bool null_or_empty(const char_type* s) {
-  return s == nullptr || *s == 0;
+  return s == NULL || *s == 0;
 }
 
 // Returns true if the path starts with a drive letter, e.g. "c:".