fix validation of web push URLs

They are validated by test message, but it would have been possible
to add an http url.

If an http url was added, it's still possible to remove it via
NS PUSH DELETE.

irc/handlers.go

@@ -3934,6 +3934,7 @@ func webpushHandler(server *Server, client *Client, msg ircmsg.Message, rb *Resp
 
 	if err := webpush.SanityCheckWebPushEndpoint(endpoint); err != nil {
 		rb.Add(nil, server.name, "FAIL", "WEBPUSH", "INVALID_PARAMS", subcommand, client.t("Invalid web push URL"))
+		return false
 	}
 
 	switch subcommand {