Crash from startup window if no GRP is found
As far as I can tell, this holds to Windows and GTK altogether, and was introduced in fec7d30e.
A partial fix for the GTK startup window was accepted from !326 (merged), but I still got crashes with the last revision (manual Linux build + Windows auto-build).
To reproduce, you can start EDuke32 with -usecwd
from a directory with a standalone game that doesn't use any grp or equivalent file.