sitnl: harden strncpy() by forcing arguments to have the same length

sitnl: harden strncpy() by forcing arguments to have the same length

At the moment a strcpy() (without length check!) is performed between a
string long IFNAMSIZ bytes and one of 16 bytes. This is ok right now
because IFNAMSIZ is defined as 16, however this bit is not under our
control and may change in he future without us being warned.

For this reason, force both strings to use IFNAMSIZ as size and, since
this constant may not exist on every platform, ensure it is always
defined.

Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20190805092529.9467-2-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg18722.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>