Don't attempt to load a password quality verifier from NULL
When kadm5_add_passwd_quality_verifier is called with a NULL
check_library parameter and [password_quality].policy_libraries
is set, the function calls add_verifier() for each string in the
policy_libraries section and then falls through to the non-NULL
case and calls add_verifier() a final time with a NULL argument.
This leads to dlopening the running executable and then failing
since it contains no password quality verifier.
If the check_library argument is NULL, only call add_verifier()
for the configured policy_libraries and do not fall through to
the non-NULL case.
Signed-off-by: Love Hornquist Astrand <lha@h5l.org>