bugfix: fix protobuf decode repeated string

this commit is picked from libpomelo

https://github.com/NetEase/libpomelo/pull/70

Thanks to @blueseason
bugfix: fix mem leak while enabling PB_DEBUG

This commit is picked from libpomelo

https://github.com/NetEase/libpomelo/commit/2f0b8d9d2c68cdfd07e483e5b6d667ef740b5b4c