layout: Range-check index before array lookup

findMonitorForActor() may be called before the layoutManager gets
to initialize monitors, so make sure the monitor index is in range
to avoid a warning.

https://bugzilla.gnome.org/show_bug.cgi?id=781471

js/ui/layout.js

@@ -896,7 +896,10 @@ const LayoutManager = new Lang.Class({
     },
 
     findMonitorForActor: function(actor) {
-        return this.monitors[this.findIndexForActor(actor)];
+        let index = this.findIndexForActor(actor);
+        if (index >= 0 && index < this.monitors.length)
+            return this.monitors[index];
+        return null;
     },
 
     _queueUpdateRegions: function() {