winbind: protect a pending wb_child_request against a talloc_free()

winbind: protect a pending wb_child_request against a talloc_free()

If the (winbind) client gave up we call TALLOC_FREE(state->mem_ctx)
in remove_client(). This triggers a recursive talloc_free() for all
in flight requests.

In order to maintain the winbindd parent-child protocol, we need
to keep the orphaned wb_simple_trans request until the parent
got the response from the child.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13290

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>