[SSSD] [PATCH] IPA: Use function sysdb_attrs_get_el in safe way

[Lukas Slebodnik]

ehlo,

Intention of ticket @2284 was to refactor function sysdb_attrs_get_el
This would be a big change and it would not ne easy to backport patch
to the branch sssd-1-11. The attached patch fixes prolem in simple way
and patch can be easily applied on top of branch sssd-1-11

Function sysdb_attrs_get_el can enlarge array of ldb_message_element in "struct
sysdb_attrs" if attribute is not among available attributes. Array will be
enlarged with function talloc_realloc but realloc can move array to another
place in memory therefore ldb_message_element should not be used after next
call of function sysdb_attrs_get_el

    sysdb_attrs_get_el(netgroup, SYSDB_ORIG_MEMBER_USER, &user_found);
    sysdb_attrs_get_el(netgroup, SYSDB_ORIG_MEMBER_HOST, &host_found);
With netgroups, it is common to omit user or host from netgroup triple.
There is very high probability that realloc will be called. it is possible
pointer user_found can refer to the old area after the second call of function
sysdb_attrs_get_el.

Resolves:
https://fedorahosted.org/sssd/ticket/2284

How to test?
sh-4.2$ getent netgroup netgroup_user1
netgroup_user2        (-,usersssd01,example.com)

and run backend with valgrind

Result:
    --without patch: there are errors like in description of ticket
                        https://fedorahosted.org/sssd/ticket/2284
    --with patch: errors are gone

LS