BUG/MINOR: server: fix race on error path of 'add server' CLI if track

If an error occurs during a dynamic server creation with tracking, it
must be removed from the tracked list. This operation is not thread-safe
and thus must be conducted under the thread isolation.

Track support for dynamic servers has been introduced in this release.
This does not need to be backported.
diff --git a/src/server.c b/src/server.c
index 5fa3946..7bef5bf 100644
--- a/src/server.c
+++ b/src/server.c
@@ -4589,15 +4589,16 @@
 	return 0;
 
 out:
+	if (srv && srv->track)
+		release_server_track(srv);
+
 	thread_release();
 
 	if (!usermsgs_empty())
 		cli_err(appctx, usermsgs_str());
 
-	if (srv) {
-		release_server_track(srv);
+	if (srv)
 		free_server(srv);
-	}
 
 	return 1;
 }