rtp sout: fix another race condition in RTSP

When RTSP is shut down, the server destroys sessions (with no locking)
while clients are still able to concurrently access them, potentially
leading to a crash. Make sure we unregister the RTSP URL before
cleaning up (then indeed no locking is needed).

modules/stream_out/rtsp.c

@@ -122,12 +122,12 @@ error:
 
 void RtspUnsetup( rtsp_stream_t *rtsp )
 {
-    while( rtsp->sessionc > 0 )
-        RtspClientDel( rtsp, rtsp->sessionv[0] );
-
     if( rtsp->url )
         httpd_UrlDelete( rtsp->url );
 
+    while( rtsp->sessionc > 0 )
+        RtspClientDel( rtsp, rtsp->sessionv[0] );
+
     if( rtsp->host )
         httpd_HostDelete( rtsp->host );