avcodec/mjpegdec: Improve intel jpeg flip heuristic

Fixes Ticket3698
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mjpegdec.c

@@ -1745,7 +1745,7 @@ static int mjpeg_decode_com(MJpegDecodeContext *s)
                 parse_avid(s, cbuf, len);
             } else if (!strcmp(cbuf, "CS=ITU601"))
                 s->cs_itu601 = 1;
-            else if ((!strncmp(cbuf, "Intel(R) JPEG Library, version 1", 32)) ||
+            else if ((!strncmp(cbuf, "Intel(R) JPEG Library, version 1", 32) && s->avctx->codec_tag) ||
                      (!strncmp(cbuf, "Metasoft MJPEG Codec", 20)))
                 s->flipped = 1;