proxy.c: Clear sensitive data after use

proxy.c: Clear sensitive data after use

Usage of credentials  is a bit odd in this file.
Actually the copy of "struct user_pass" kept in p->up is not
required at all. It just defeats the purpose of auth-nocahe
as it never gets cleared.

Removing it is beyond the scope of this patch -- we just ensure
it's purged after use.

Change-Id: Ic6d63a319d272a56ac0e278f1356bc5241b56a34
Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20240905100724.4105-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg29061.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>