wayland/xdg-toplevel-drag: Fix grab actor setup regression

If called before clutter_grab_activate, clutter_stage_get_grab_actor returns NULL. This fixes the order of the calls and adds a safe-guard to check the returned grab_actor is non-null, which in theory may (unlikely) fail.

Closes: #3877

Signed-off-by: Nick Diego Yamane nickdiego@igalia.com