avcodec/mpegvideo_enc: Replace some s->avctx by avctx

avcodec/mpegvideo_enc: Replace some s->avctx by avctx

Fixes issue reported by: Xu Guangxin <guangxin.xu@intel.com>
Original report:
Steps to reproduce:
1. ./configure --enable-debug=3 --disable-libx264 && make install
2. ffmpeg -i input.mp4 -profile:v baseline output.mp4 -y

you will see a crash like this:
[mpeg4 @ 0x5555575854c0] [Eval @ 0x7fffffffbf80] Undefined constant or missing '(' in 'baseline'
[mpeg4 @ 0x5555575854c0] Unable to parse option value "baseline"
[mpeg4 @ 0x5555575854c0] Error setting option profile to value baseline.
Thread 1 "ffmpeg" received signal SIGSEGV, Segmentation fault.

root cause:
If the codec has FF_CODEC_CAP_INIT_CLEANUP flag, and avcodec_open2 got an error before avctx->codec->init,
the ff_mpv_encode_end will face a null s->avctx.

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>