Add error-checking for the case where _arg_cache_new() fails
Add error-checking for the case where _arg_cache_new() fails

This can happen when a typelib and its underlying library are out-of-sync. This
converts the segfault into a more helpful traceback.

https://bugzilla.gnome.org/show_bug.cgi?id=678914

davidmalcolm/pygobject,MathieuDuponchelle/pygobject,thiblahute/pygobject,MathieuDuponchelle/pygobject,davibe/pygobject,GNOME/pygobject,davibe/pygobject,pexip/pygobject,davibe/pygobject,sfeltman/pygobject,sfeltman/pygobject,thiblahute/pygobject,GNOME/pygobject,davibe/pygobject,pexip/pygobject,davidmalcolm/pygobject,davidmalcolm/pygobject,MathieuDuponchelle/pygobject,GNOME/pygobject,pexip/pygobject,thiblahute/pygobject,sfeltman/pygobject