mpegvideo_parser: fix buffer access beyond end

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

libavcodec/mpegvideo_parser.c

@@ -56,6 +56,7 @@ static void mpegvideo_extract_headers(AVCodecParserContext *s,
         case PICTURE_START_CODE:
             if (bytes_left >= 2) {
                 s->pict_type = (buf[1] >> 3) & 7;
+                if (bytes_left >= 4)
                 vbv_delay = ((buf[1] & 0x07) << 13) | (buf[2] << 5) | (buf[3]  >> 3);
             }
             break;