Don't follow RTSP redirects when used as a muxer

Originally committed as revision 21966 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/rtsp.c

@@ -1543,7 +1543,7 @@ redirect:
  fail:
     rtsp_close_streams(s);
     url_close(rt->rtsp_hd);
-    if (reply->status_code >=300 && reply->status_code < 400) {
+    if (reply->status_code >=300 && reply->status_code < 400 && s->iformat) {
         av_strlcpy(s->filename, reply->location, sizeof(s->filename));
         av_log(s, AV_LOG_INFO, "Status %d: Redirecting to %s\n",
                reply->status_code,