set AVFrame decode_error_flags in case h->slice_ctx->er.error_occurred is set
after the call to ff_h264_execute_decode_slices. This allows the user to detect
concealed decoding errors in the call to avcodec_receive_frame
Signed-off-by: Amir Pauker <amir@livelyvideo.tv>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>