avcodec/mpeg12dec: Fix got_output

avcodec/mpeg12dec: Fix got_output

This makes got_output consistent with the code in slice_end() which sets the output

in slice_end()
 if (s->pict_type == AV_PICTURE_TYPE_B || s->low_delay) {
     int ret = av_frame_ref(pict, s->current_picture_ptr->f);
     ...
 } else {

Fixes: assertion failure
Fixes: 22178/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MPEG1VIDEO_fuzzer-5664234440753152
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>