Handle NULL returns from glibc 2.17+ crypt().

Starting with glibc 2.17 (eglibc 2.17), crypt() fails with EINVAL
(w/ NULL return) if the salt violates specifications. Additionally,
on FIPS-140 enabled Linux systems, DES/MD5-encrypted passwords
passed to crypt() fail with EPERM (w/ NULL return).

If using glibc's crypt(), check return value to avoid a possible
NULL pointer dereference.