rtsp: Only set the ttl parameter if the server actually gave a value

Passing ttl=0 to the rtp/udp url contexts makes packets never
leave the host machine.

Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/rtsp.c

@@ -1350,7 +1350,7 @@ int ff_rtsp_make_setup_request(AVFormatContext *s, const char *host, int port,
             break;
         }
         case RTSP_LOWER_TRANSPORT_UDP_MULTICAST: {
-            char url[1024], namebuf[50];
+            char url[1024], namebuf[50], optbuf[20] = "";
             struct sockaddr_storage addr;
             int port, ttl;
 
@@ -1363,10 +1363,12 @@ int ff_rtsp_make_setup_request(AVFormatContext *s, const char *host, int port,
                 port      = rtsp_st->sdp_port;
                 ttl       = rtsp_st->sdp_ttl;
             }
+            if (ttl > 0)
+                snprintf(optbuf, sizeof(optbuf), "?ttl=%d", ttl);
             getnameinfo((struct sockaddr*) &addr, sizeof(addr),
                         namebuf, sizeof(namebuf), NULL, 0, NI_NUMERICHOST);
             ff_url_join(url, sizeof(url), "rtp", NULL, namebuf,
-                        port, "?ttl=%d", ttl);
+                        port, "%s", optbuf);
             if (ffurl_open(&rtsp_st->rtp_handle, url, AVIO_FLAG_READ_WRITE,
                            &s->interrupt_callback, NULL) < 0) {
                 err = AVERROR_INVALIDDATA;