* src/groupmems.c: When removing an user, check if deluser is on

	* src/groupmems.c: When removing an user, check if deluser is on
	the list, not adduser. This fixes a segmentation fault for every
	call of groupmems -d.
	* libmisc/list.c: Add assertions to help identifying these issues.
	* libmisc/list.c: Avoid implicit conversion of pointers to
	booleans.