dateMenu: Ignore malformed world-clocks settings

Guard against a malformed setting that doesn't contain a
'location' key.

https://bugzilla.gnome.org/show_bug.cgi?id=791148

js/ui/dateMenu.js

@@ -132,6 +132,8 @@ var WorldClocksSection = new Lang.Class({
         let world = GWeather.Location.get_world();
         let clocks = settings.get_value('world-clocks').deep_unpack();
         for (let i = 0; i < clocks.length; i++) {
+            if (!clocks[i].location)
+                continue;
             let l = world.deserialize(clocks[i].location);
             this._locations.push({ location: l });
         }