wayland: guard null clipboard offer

[brahmkshatriya]

• I confirm that I am the author of this code and release it to the SDL project under the zlib license. This contribution does not contain code from other sources, including code generated by a Large Language Model ("AI").

Description

The Wayland selection callback can invoke Wayland_DataOfferNotifyFromMIMEs(NULL, ...) when the compositor clears the clipboard selection. The function already handles a null offer while collecting MIME types, but subsequently passed that null pointer unconditionally to SetCurrentClipboardOffer(), which dereferences it.

This caused a reproducible segmentation fault in the Wayland event pump:

Wayland_DataOfferNotifyFromMIMEs
libffi
libwayland-client
Wayland_PumpEvents
SDL_WaitEventTimeoutNS

Existing Issue(s)

No existing issue found for this specific null dereference.

[Kontrabant]

Hmm, this fixes the issue on the surface, but there's still state that should be updated, even with a nil offer, that this would prevent. I think I have a better way to handle this.

[brahmkshatriya]

Oh? what would that be? to update the clipboard state using the data device/seat even when the offer is nil?