Fix workspace manager cleanup with mutter 3.3x

fix-workspace-manager-cleanup.patch

diff --git a/src/WorkspaceManager.vala b/src/WorkspaceManager.vala
index dd4a09e..c49fe4c 100644
--- a/src/WorkspaceManager.vala
+++ b/src/WorkspaceManager.vala
@@ -389,9 +389,13 @@ namespace Gala
 #if HAS_MUTTER330
             unowned Meta.Display display = wm.get_display ();
 			unowned Meta.WorkspaceManager manager = display.get_workspace_manager ();
-			var last_index = manager.get_n_workspaces () - 1;
+			unowned List<Meta.Workspace> existing_workspaces = null;
 			for (int i = 0; i < manager.get_n_workspaces (); i++) {
-			    unowned Meta.Workspace workspace = manager.get_workspace_by_index (i);
+				existing_workspaces.append (manager.get_workspace_by_index (i));
+			}
+
+			foreach (var workspace in workspaces) {
+				var last_index = manager.get_n_workspaces () - 1;
 				if (Utils.get_n_windows (workspace) < 1
 					&& workspace.index () != last_index) {
 					remove_workspace (workspace);