avcodec/mjpegdec: Print error in case of CMYK

Also fail if AV_EF_EXPLODE is set.
We do not fail by default, but rather return some image as it may be usefull to the
end user to see what is on the image, for example text could be read quite fine and
objects recognized.

Possibly fixes Ticket3424
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mjpegdec.c

@@ -378,6 +378,11 @@ int ff_mjpeg_decode_sof(MJpegDecodeContext *s)
         if (s->rgb)
             s->avctx->pix_fmt = s->bits <= 9 ? AV_PIX_FMT_ABGR : AV_PIX_FMT_RGBA64;
         else {
+            if (s->adobe_transform == 0) {
+                av_log(s->avctx, AV_LOG_ERROR, "CMYK Unsupported\n");
+                if (s->avctx->err_recognition & AV_EF_EXPLODE)
+                    goto unk_pixfmt;
+            }
             s->avctx->pix_fmt = s->bits <= 8 ? AV_PIX_FMT_YUVA444P : AV_PIX_FMT_YUVA444P16;
             s->avctx->color_range = s->cs_itu601 ? AVCOL_RANGE_MPEG : AVCOL_RANGE_JPEG;
         }
@@ -1836,6 +1841,7 @@ int ff_mjpeg_decode_frame(AVCodecContext *avctx, void *data, int *got_frame,
 
     av_dict_free(&s->exif_metadata);
     av_freep(&s->stereo3d);
+    s->adobe_transform = -1;
 
     buf_ptr = buf;
     buf_end = buf + buf_size;