os: Fix open result check.

Fix defect reported by Coverity Scan.

Argument cannot be negative (NEGATIVE_RETURNS) negative_returns: f is passed to a parameter that cannot be negative.

Fixes: 13fa0513 ("auxiliary/os: add new os_get_command_line() function") Signed-off-by: Vinson Lee vlee@freedesktop.org

cc: @brianp