tun.c: fix 'use after free' error

tun.c: fix 'use after free' error

Commit 509c45f has factored out code blocks of open_tun()
into separate functions and introduced "use after free" bug:

Variable "device_guid" is allocated inside tun_open_device()
function and used outside of it. Allocation happens with
local gc_arena, which is freed at the end of tun_open_device(),
making futher access to "device_guid" invalid.

Fix by ensuring that gc_arena scope covers all access to "device_guid".

Signed-off-by: Lev Stipakov <lev@openvpn.net>
Acked-by: Simon Rozman <simon@rozman.si>
Message-Id: <20200312060829.19468-1-lstipakov@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg19547.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>