workspacesView: Initialize the adjustment's upper bound

Initializing the upper bound to zero means that on panning we'd start
scrolling from the first workspace even if the current workspace when
entering the overview was different since StAdjustment clamps the
value to be inside bounds.

https://bugzilla.gnome.org/show_bug.cgi?id=766883

js/ui/workspacesView.js

@@ -103,7 +103,7 @@ const WorkspacesView = new Lang.Class({
                                                     page_increment: 1,
                                                     page_size: 1,
                                                     step_increment: 0,
-                                                    upper: 0 });
+                                                    upper: global.screen.n_workspaces });
         this.scrollAdjustment.connect('notify::value',
                                       Lang.bind(this, this._onScroll));