avformat/libzmq: fix check for zmq protocol prefix

Fixes ticket #11134.

Signed-off-by: Marton Balint <cus@passwd.hu>
(cherry picked from commit a87a96105e)

libavformat/libzmq.c

@@ -94,7 +94,7 @@ static int zmq_proto_open(URLContext *h, const char *uri, int flags)
         return AVERROR_EXTERNAL;
     }
 
-    if (av_strstart(uri, "zmq:", &uri)) {
+    if (!av_strstart(uri, "zmq:", &uri)) {
         av_log(h, AV_LOG_ERROR, "URL %s lacks prefix\n", uri);
         return AVERROR(EINVAL);
     }