segv in debug_write_type

In commit 6c3458a8b7ee I claimed that u.kenum is always non-NULL,
which is true for debug_make_enum_type, but not for enums made by
debug_make_undefined_tagged_type.  Fix that oversight

	PR 32829
	* debug.c (debug_write_type): Handle NULL u.kenum.
	(debug_type_samep): Likewise.