Fix a crash in pyg_type_add_interfaces()
Fix a crash in pyg_type_add_interfaces()

Reported as part of bug #566571.

davibe/pygobject,choeger/pygobject-cmake,davidmalcolm/pygobject,davibe/pygobject,davidmalcolm/pygobject,thiblahute/pygobject,davibe/pygobject,davidmalcolm/pygobject,GNOME/pygobject,MathieuDuponchelle/pygobject,choeger/pygobject-cmake,nzjrs/pygobject,pexip/pygobject,MathieuDuponchelle/pygobject,alexef/pygobject,Distrotech/pygobject,nzjrs/pygobject,jdahlin/pygobject,thiblahute/pygobject,MathieuDuponchelle/pygobject,Distrotech/pygobject,sfeltman/pygobject,pexip/pygobject,thiblahute/pygobject,alexef/pygobject,alexef/pygobject,davibe/pygobject,sfeltman/pygobject,GNOME/pygobject,choeger/pygobject-cmake,Distrotech/pygobject,nzjrs/pygobject,pexip/pygobject,sfeltman/pygobject,jdahlin/pygobject,jdahlin/pygobject,Distrotech/pygobject,GNOME/pygobject