Corrected LDAP bug

This bug allowed users to log in even if no password was entered.

I have not tested this code but users say it solved their problems... Can you please take a look?

Code by drizzt at http://dolibarr.es/index.php/foro/7-bugs-versiones-estables/4152-error-en-validacion-contra-ldapcontrasena-vacia?limit=6&start=12#4209