avcodec/mjpegbdec: Check for AVDISCARD_ALL

Fixes: Assertion failure
Fixes: 51825/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MJPEGB_fuzzer-6393802688692224

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mjpegbdec.c

@@ -141,9 +141,10 @@ read_header:
         av_log(avctx, AV_LOG_WARNING, "no picture\n");
         return buf_size;
     }
-
     av_frame_move_ref(rframe, s->picture_ptr);
     s->got_picture = 0;
+    if (avctx->skip_frame == AVDISCARD_ALL)
+        return AVERROR(EAGAIN);
     *got_frame = 1;
 
     if (!s->lossless && avctx->debug & FF_DEBUG_QP) {