phase2_tcp_server: fix Coverity issue 'Dereference after null check'

phase2_tcp_server: fix Coverity issue 'Dereference after null check'

As Coverity says:
Either the check against null is unnecessary, or there may be a null
pointer dereference.
In phase2_tcp_server: Pointer is checked against null but then
dereferenced anyway

There is only one caller (link_socket_init_phase2) and it already has
an ASSERT(sig_info). So use that here was well.

v2:
 - fix cleanly by actually asserting that sig_info is defined

Change-Id: I8ef199463d46303129a3f563fd9eace780a58b8a
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Arne Schwabe <arne-openvpn@rfc2549.org>
Message-Id: <20240325071448.12143-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg28452.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit e8c629fe64c67ea0a8454753be99db44df7ce53e)