jpegdec: Only enable rgb mode when there are 3 components.

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

libavcodec/mjpegdec.c

@@ -279,7 +279,7 @@ int ff_mjpeg_decode_sof(MJpegDecodeContext *s)
         return -1;
     }
 
-    if(s->v_max==1 && s->h_max==1 && s->lossless==1) s->rgb=1;
+    if(s->v_max==1 && s->h_max==1 && s->lossless==1 && nb_components==3) s->rgb=1;
 
     /* if different size, realloc/alloc picture */
     /* XXX: also check h_count and v_count */