Fix stack buffer overruns in NEXTADDR() macro:

Fix stack buffer overruns in NEXTADDR() macro:

copy first, then round up the length when adding padding
to the advance.

Found by: GCC 9.3.0 (FreeBSD)

Signed-off-by: Matthias Andree <matthias.andree@gmx.de>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20200717171818.230371-1-matthias.andree@gmx.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg20461.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 5fde831c580775aa5c1fe3539b06260d994eee10)