nsswitch/wb_common.c: don't operate on a stale wb_global_ctx.key

nsswitch/wb_common.c: don't operate on a stale wb_global_ctx.key

If nss_winbind is loaded into a process that uses fork multiple times
without any further calls into nss_winbind, wb_atfork_child handler
was using a wb_global_ctx.key that was no longer registered in the
pthread library, so we operated on a slot that was potentially
reused by other libraries or the main application. Which is likely
to cause memory corruption.

So we better don't call pthread_key_delete() in wb_atfork_child().

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15464

Reported-by: Krzysztof Piotr Oledzki <ole@ans.pl>
Tested-by: Krzysztof Piotr Oledzki <ole@ans.pl>
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>