Fix access to data that might be deleted
PPS data in reference pictures are deleted if a PPS with the same PPS_id have been sent, and the access to getMixedNaluTypesInPicFlag() is not valid any more.
PPS data in reference pictures are deleted if a PPS with the same PPS_id have been sent, and the access to getMixedNaluTypesInPicFlag() is not valid any more.