Only update frame calculation if we have a valid link sockets

Only update frame calculation if we have a valid link sockets

Without this, we will caculate a pointer to the linksocket relative to a
null pointer in get_link_socket_info(), which itself does not crash and
the pointer seems not to be accessed later, so we do not get a crash here.

This is still not the correct behaviour and the undefined behaviour
sanitiser from llvm/clang finds this.

Change-Id: I82a20ac72f60f8770ea1b4ab0c8cdea31868abe7
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20230301134455.2810114-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg26318.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>