avcodec/mjpegdec: Only allow 0x11110000 pix_fmt_id for bayer mode

Fixes: NULL pointer dereference
Fixes: assertion failure
Fixes: 17003/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MJPEG_fuzzer-5696929253556224
Fixes: 17039/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MJPEG_fuzzer-5651008134316032

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

libavcodec/mjpegdec.c

@@ -501,6 +501,8 @@ int ff_mjpeg_decode_sof(MJpegDecodeContext *s)
 
         switch (pix_fmt_id) {
         case 0x11110000: /* for bayer-encoded huffman lossless JPEGs embedded in DNGs */
+            if (!s->bayer)
+                goto unk_pixfmt;
             s->avctx->pix_fmt = AV_PIX_FMT_GRAY16LE;
             break;
         case 0x11111100: