Skip over LDAP group members that don't exist

This tickles a bug as reported by Thorsten Marquardt in https://sourceforge.net/p/davical/mailman/message/58766531/ with this error message reported:

Exception [0] ldap_get_attributes(): Argument #2 ($entry) must be of type LDAP\ResultEntry, false given
At line 718 of /usr/share/davical-master/inc/drivers_ldap.php

You need to look in 3da860e5 for that line number as I've just made some improvements to the code the last few commits.