avcodec/mjpegbdec: Skip SOS on AVDISCARD_ALL as does mjpeg

Fixes: NULL pointer dereference
Fixes: 36342/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MJPEGB_fuzzer-4579188072906752
Fixes: 36344/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MJPEGB_fuzzer-5049579300061184
Fixes: 36345/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MJPEGB_fuzzer-5301149845553152
Fixes: 36374/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MJPEGB_fuzzer-6056312352931840

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

@@ -119,9 +119,13 @@ read_header:
                       8 * FFMIN(field_size, buf_end - buf_ptr - sos_offs));
         s->mjpb_skiptosod = (sod_offs - sos_offs - show_bits(&s->gb, 16));
         s->start_code = SOS;
-        ret = ff_mjpeg_decode_sos(s, NULL, 0, NULL);
-        if (ret < 0 && (avctx->err_recognition & AV_EF_EXPLODE))
-            return ret;
+        if (avctx->skip_frame == AVDISCARD_ALL) {
+            skip_bits(&s->gb, get_bits_left(&s->gb));
+        } else {
+            ret = ff_mjpeg_decode_sos(s, NULL, 0, NULL);
+            if (ret < 0 && (avctx->err_recognition & AV_EF_EXPLODE))
+                return ret;
+        }
     }
 
     if (s->interlaced) {