Fix workspaces accumulating on each restart.

Main._nWorkspacesChanged was racing with Main._checkWorkspaces.
If _checkWorkspaces won the race, _workspaces was uninitialized.
Because of this, _checkWorkspaces only noticed workspaces
with windows on them, leading it to believe the last workspace wasn't
empty, and added a new, empty workspace.

https://bugzilla.gnome.org/show_bug.cgi?id=645343

js/ui/main.js

@@ -223,7 +223,7 @@ function start() {
     global.screen.connect('window-entered-monitor', _windowEnteredMonitor);
     global.screen.connect('window-left-monitor', _windowLeftMonitor);
 
-    Mainloop.idle_add(_nWorkspacesChanged);
+    _nWorkspacesChanged();
 }
 
 let _workspaces = [];