wayland: Prepare against NULL selection owners on data_source requests

wayland: Prepare against NULL selection owners on data_source requests

Currently, due to the lack of progress information in the Wayland DnD
protocol, we assume a DnD operation is finished after the first
data_source.send is finished (It's either that or leaving stuck grabs).

This however breaks previous assumptions that dest widgets can request
the data multiple times, even in response to GtkWidget::drag-motion.
This leaves us with a NULL owner for the DnD atom when we aren't
finished receiving wl_data_source events yet, causing a crash.

This commit fixes the crash, the behavior left is still far from
desirable though...