Resolve vanadium/issues#1390

When discover.Advertise returns an error, the "stop" function is nil.
Don't call it.

Change-Id: I04f3c41f180957723afb393a19f5d7dae70132d0
diff --git a/lib/discovery/advertise_server.go b/lib/discovery/advertise_server.go
index 9e9a73a..206a170 100644
--- a/lib/discovery/advertise_server.go
+++ b/lib/discovery/advertise_server.go
@@ -37,7 +37,6 @@
 	curAddrs := sortedNames(status.Endpoints)
 	stop, err := advertiseServer(ctx, d, ad, curAddrs, suffix, visibility)
 	if err != nil {
-		stop()
 		return nil, err
 	}