udp: Properly check for invalid sockets

If stdin has been closed, 0 is a valid socket descriptor.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/udp.c

@@ -200,7 +200,7 @@ static int udp_socket_create(UDPContext *s, struct sockaddr_storage *addr,
         goto fail;
     for (res = res0; res; res=res->ai_next) {
         udp_fd = socket(res->ai_family, SOCK_DGRAM, 0);
-        if (udp_fd > 0) break;
+        if (udp_fd != -1) break;
         log_net_error(NULL, AV_LOG_ERROR, "socket");
     }