LDAP: do not invalidate pointer with realloc while processing ghost users
One peculiarity of the sysdb_attrs_get_el interface is that if the
attribute does not exist, then the attrs array is reallocated and the
element is created. But in case other pointers are already pointing
into the array, the realloc might invalidate them.
Such case was in the sdap_process_ghost_members function where if
the group had no members, the "gh" pointer requested earlier might have
been invalidated by the realloc in order to create the member element.