reliable: Review and fix gc_arena usage

reliable: Review and fix gc_arena usage

Check for unused objects (in
reliable_get_num_output_sequenced_available)
and missing free (in reliable_can_get).

While looking through the code, modernize
the loop variable usage.

Change-Id: I8cefa9a406fe90bb3cbe481304782c639691a3a0
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Arne Schwabe <arne-openvpn@rfc2549.org>
Message-Id: <20250715143750.9719-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg32157.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>