tun: always pass a valid tt pointer

tun: always pass a valid tt pointer

This patch is a small "logic restyle" which basically moves the
check for "tt != NULL" outside of the various close_tun()
implementations and replaces it with an ASSERT.

This way the check is done only once and the function can rely
on the assumption that "tt" is always valid.

This change is mainly to improve the code style inside close_tun()
implementations by removing one level of indentation.

No functional change is present.

Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20180613122824.4207-3-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg17045.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>