Avoid relayout when not changing the workspace reserved slot

Make calling workspace.setReservedSlot(null) do nothing if the slot was
already null; this improves efficiency and more importantly chills out some
weird reentrancy at the end of drag and drop that removes a window from
a workspace.

js/ui/workspace.js

@@ -921,6 +921,9 @@ Workspace.prototype = {
     },
 
     setReservedSlot: function(clone) {
+        if (this._reservedSlot == clone)
+            return;
+
         if (clone && this.containsMetaWindow(clone.metaWindow)) {
             this._reservedSlot = null;
             this.positionWindows(WindowPositionFlags.ANIMATE);