dash: Make sure _clearDragPlaceholder() resets _dragPlaceholderPos

The function currently only resets the placeholder position if
there is a placeholder; this is not necessarily true, as the
placeholder may be reset outside _clearDragPlaceholder().
If this happens, the placeholder will temporarily stop working
for the "old" position (and permanently if it's the only position).
Just reset the position unconditionally.

https://bugzilla.gnome.org/show_bug.cgi?id=684618

js/ui/dash.js

@@ -799,8 +799,8 @@ const Dash = new Lang.Class({
         if (this._dragPlaceholder) {
             this._dragPlaceholder.animateOutAndDestroy();
             this._dragPlaceholder = null;
-            this._dragPlaceholderPos = -1;
         }
+        this._dragPlaceholderPos = -1;
     },
 
     handleDragOver : function(source, actor, x, y, time) {