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
20afd3a63a75a160f61a98a8dcfe06f527ea19b4)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>