avcodec/mjpegdec: Check for rgb before flipping

Fixes assertion failure due to unsupported case

Fixes: 356/fuzz-1-ffmpeg_VIDEO_AV_CODEC_ID_MJPEG_fuzzer
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mjpegdec.c

@@ -2386,7 +2386,7 @@ the_end:
             }
         }
     }
-    if (s->flipped) {
+    if (s->flipped && !s->rgb) {
         int j;
         avcodec_get_chroma_sub_sample(s->avctx->pix_fmt, &hshift, &vshift);
         av_assert0(s->nb_components == av_pix_fmt_count_planes(s->picture_ptr->format));