ibusManager: Fix a warning

Don't try to access a non-existent engine - it probably makes sense to
use Map() instead of a plain object to track engines in the future, but
for now just add an additional check to shut up a warning.

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

js/misc/ibusManager.js

@@ -190,7 +190,7 @@ const IBusManager = new Lang.Class({
     },
 
     getEngineDesc: function(id) {
-        if (!IBus || !this._ready)
+        if (!IBus || !this._ready || !this._engines.hasOwnProperty(id))
             return null;
 
         return this._engines[id];