Fix compiler warning in reliable.c with --disable-debug

Fix compiler warning in reliable.c with --disable-debug

Use the easy way out. Using pre-compiler to completely
avoid n_active seems like overkill.

Change-Id: Icad1a52d14311a6f06bda081cab2f4bded8d47ed
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Arne Schwabe <arne-openvpn@rfc2549.org>
Message-Id: <20250711100405.240625-1-frank@lichtenheld.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg32107.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit fcd8f0f9fccdba02ed643dee5df7422ba36ced12)