libnm,vpn-service-plugin: remove old connect timer when adding new one

If the plugin didn't succeed connecting once, don't let the old timer fire
during a subsequent connection.

https://bugzilla.redhat.com/show_bug.cgi?id=1271973

libnm/nm-vpn-service-plugin.c

@@ -424,6 +424,7 @@ connect_timer_start (NMVpnServicePlugin *plugin)
 {
 	NMVpnServicePluginPrivate *priv = NM_VPN_SERVICE_PLUGIN_GET_PRIVATE (plugin);
 
+	nm_clear_g_source (&priv->connect_timer);
 	priv->connect_timer = g_timeout_add_seconds (60, connect_timer_expired, plugin);
 }