gdk: fix crash on Wayland with already destroyed surface.

See crash reliably reproduced on GIMP when a popup is destroyed on a drag-begin callback and Wayland data listener callbacks calls the enter() callback with a NULL surface.

Cf. gimp#10515 (closed)

This is the GTK4/main branch variant for !8016 (merged) (which fixes the same bug for GIMP 3 on GTK3).