LDAP: avoid complex realloc logic in save_rfc2307bis_group_memberships
    
    https://fedorahosted.org/sssd/ticket/1761
    
    The function tried to be smart and realloc only when needed, but that
    only lead to hard-to find bugs where the logic would not allocate the
    proper space. Remove the reallocation and prefer readability over speed
    in this case.