Move pre decrypt lite check to its own function

Move pre decrypt lite check to its own function

This prepares for extending this function with the HMAC based session ID
check.

Replace the check for m->top.c2.tls_auth_standalone with an ASSERT as this
code path is only used in multi udp server and OpenVPN initialises the
tls_auth_standalone always for the TOP context (CF_INIT_TLS_AUTH_STANDALONE),
even for the tcp m2mp server that does not use it).

Patch v2: replace if with ASSERT

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20220425122709.4148015-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg24193.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>