PUSH_UPDATE server: check IV_PROTO before sending the message to the client

PUSH_UPDATE server: check IV_PROTO before sending the message to the client

Before sending the PUSH_UPDATE message to the client, we must verify that
the client has actually sent IV_PROTO_PUSH_UPDATE to the server, declaring that
it supports push-updates.

Also fixed a gc_arena memory leak in one of the error paths and asserted
mi->context.c2.tls_multi .

Change-Id: I7c28da72be11c7efbed3068fbfc65f2959227bec
Signed-off-by: Marco Baffo <marco@mandelbit.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1255
Message-Id: <20251009182855.18712-1-gert@greenie.muc.de>
URL: https://sourceforge.net/p/openvpn/mailman/message/59244566/
Signed-off-by: Gert Doering <gert@greenie.muc.de>