[3.13] gh-140734: fix off-by-one error when comparing to `_SUN_PATH_MAX` (GH-140903) (#141182)

gh-140734: fix off-by-one error when comparing to `_SUN_PATH_MAX` (GH-140903)

The limit includes a NULL terminator.
(cherry picked from commit 9a199006733dae999f96c0f596c2035f4b9847b2)

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>