auth_token.c: add NULL initialization

auth_token.c: add NULL initialization

This fixes

  error C4703: potentially uninitialized local pointer variable
'b64output' used

found by arm64 msvc compiler with SDL enabled.

Not sure why this is not triggered on x86/x64.

Signed-off-by: Lev Stipakov <lev@openvpn.net>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20220107123550.188-1-lstipakov@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23511.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>