network: Avoid JS error when activating VPN connection

If the call to settings.get_connection_by_path in
ensureActiveConnectionProps returns null, we'll hit a JS error here.
Seems to happen always when activating a VPN connection. Avoid that.

Giovanni says:

"I believe this is papering over an existing bug, but it's possible for
settings.get_connection_by_path() to legitimately return null (if the
connection is owned by a different user and invisible to the current
one), so the fix is correct anyway."

https://bugzilla.gnome.org/show_bug.cgi?id=759793

js/ui/status/network.js

@@ -1543,8 +1543,10 @@ const NMVPNSection = new Lang.Class({
             item.setActiveConnection(null);
         }
         vpnConnections.forEach(Lang.bind(this, function(a) {
+            if (a._connection) {
                 let item = this._connectionItems.get(a._connection.get_uuid());
                 item.setActiveConnection(a);
+            }
         }));
     },