mjpeg: Detect overreads in mjpeg_decode_scan() and error out.

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

libavcodec/mjpegdec.c

@@ -797,6 +797,10 @@ static int mjpeg_decode_scan(MJpegDecodeContext *s, int nb_components, int Ah, i
             if (s->restart_interval && !s->restart_count)
                 s->restart_count = s->restart_interval;
 
+            if(get_bits_count(&s->gb)>s->gb.size_in_bits){
+                av_log(s->avctx, AV_LOG_ERROR, "overread %d\n", get_bits_count(&s->gb) - s->gb.size_in_bits);
+                return -1;
+            }
             for(i=0;i<nb_components;i++) {
                 uint8_t *ptr;
                 int n, h, v, x, y, c, j;