error code of crypt-blowfish
According to http://www.openwall.com/lists/oss-security/2011/11/15/3,
On encountering an error, it should return something else than ':'.
Upon discussion with sjg@, we decided to change to NULL. According to
the man page(crypt(3)), upon an error, NULL should be returned.
I tested by changing hash from sha to blf and generating new passwords.