t/op/taint.t: Perform SHA-256 algorithm by crypt() if default one is disabled

The crypt(3) call may return NULL. This is the case on FIPS-enabled
platforms. Then "tainted crypt" test would fail.

See RT#121591 for similar fix in t/op/crypt.t.

Signed-off-by: Petr Písař <ppisar@redhat.com>