SDAP: sanitize member name before using in filter
    
    It caused an errors.
    
    (Tue Aug  2 06:29:39 2016) [sssd[be[LDAP]]] [sysdb_cache_search_users]
    (0x2000): Search users with filter:
    (&(objectclass=user)(nameAlias=t(u)ser@ldap))
    (Tue Aug  2 06:29:39 2016) [sssd[be[LDAP]]] [sysdb_cache_search_users]
    (0x0080): Error: 5 (Input/output error)
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/3121
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>