delete_routes(_ipv6): avoid memleak if RT_DEFINED is not set

delete_routes(_ipv6): avoid memleak if RT_DEFINED is not set

In this case the function would exit without releasing the argv object
initialized via argv_new().

Move initialization of argv after thic check is performed.

While at it, also move the declaration of gc and initizlize it with
gc_new().

Reported-by: Camille Guérin <guerincamille56@gmail.com>
Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20220917224227.16988-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25245.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>