bug#56487: xgselect race condition leading to abort when USE_GTK not defined

[Tom Gillespie]

For the record, while I was hunting a better repro for
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=56488
I had the following abort happen when built with gtk3,
so it seems that there are other conditions that can
trigger the abort in the gtk3 code path as well. The
sequence of events is attached.

for-the-record.txt
Description: Text document