enumeration: fix talloc context
If for some reason ptask fails (e.g. timeout), req is talloc freed
but because subreq is attached to ectx which is permanent it is
finished anyway. Then a crash occures when we are trying to access
callback data.
The same happens in sdap_dom_enum_ex_send.
Resolves:
https://fedorahosted.org/sssd/ticket/2611
Reviewed-by: Pavel Reichl <preichl@redhat.com>
(cherry picked from commit 725bb2a9901c4f673b107ed179f5d68ec443ca63)
(cherry picked from commit 81bb9be1ae0b2a4ebe960f136a52576abcdfbbac)