avcodec/mxpegdec: Check for AVDISCARD_ALL

Fixes: Fixes NULL pointer dereference
Fixes: 36610/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MXPEG_fuzzer-6052641783283712
Fixes: 37907/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MXPEG_fuzzer-4725170850365440
Fixes: 37904/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MXPEG_fuzzer-6367889262247936
Fixes: 38085/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MXPEG_fuzzer-5175270823297024

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg


Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 20afd3a6)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>