window: Ignore requests to be placed on non-existent workspaces

When an X11 window requests an initial workspace, we currently trust
it that the workspace actually exists. However dynamic workspaces
make this easy to get wrong for applications: They make it likely
for the number of workspaces to change between application starts,
and if the app blindly applies its saved state on startup, it will
trigger an assertion.

Make sure that we pass valid parameters to set_workspace_state(),
and simply let the workspace assignment fall through to the default
handling otherwise.

https://gitlab.gnome.org/GNOME/mutter/issues/1029

src/core/window.c

@@ -1288,6 +1288,9 @@ _meta_window_shared_new (MetaDisplay         *display,
                                                                      window->initial_workspace);
         }
 
+      /* Ignore when a window requests to be placed on a non-existent workspace
+       */
+      if (on_all_workspaces || workspace != NULL)
         set_workspace_state (window, on_all_workspaces, workspace);
     }