Keyboard: clear currentSource after destruction

StLabel doesn't like that we set its properties after destructions,
and this would happen in currentInputSourceChanged() at the end,
when setting the ornament.

js/ui/status/keyboard.js

@@ -459,6 +459,7 @@ const InputSourceIndicator = new Lang.Class({
 
         this._inputSources = {};
         this._ibusSources = {};
+        this._currentSource = null;
 
         let inputSourcesByShortName = {};