util/futex: fix dangling pointer use

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110901


Fixes: 7dc2f478 "util: emulate futex on FreeBSD using umtx"
Cc: Greg V <greg@unrelenting.technology>
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit 9996ddbb)