selinux: update mac_selinux_free()

* Drop mac_selinux_use() condition from mac_selinux_free(): if the
  passed pointer holds memory we want to free it even if SELinux is
  disabled

* Drop NULL-check cause man:freecon(3) states that freecon(NULL) is a
  well-defined NOP

* Assert that on non-SELinux builds the passed pointer is always NULL,
  to avoid memory leaks