Fix xwayland DND crash

Drag and drop objects slightly outlive wayland's DND concept as we have
to cancel the client and wait for a response.

This normally is fine, except in the case that the drag ended because
the sender quit.

Calling setWlSource on drag ends creates a matching pair with
Dnd::startDrag where we first set the source and has parralels with
clipboard.

Selection::handleSelectionRequest checks for the presence of a source.

I could not reproduce the original bug.

BUG: 448920