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>

libavcodec/mxpegdec.c

@@ -193,6 +193,9 @@ static int mxpeg_decode_frame(AVCodecContext *avctx,
     int start_code;
     int ret;
 
+    if (avctx->skip_frame == AVDISCARD_ALL)
+        return AVERROR_PATCHWELCOME;
+
     buf_ptr = buf;
     buf_end = buf + buf_size;
     jpg->got_picture = 0;