MFC r218019: Do not trip a KASSERT if /dev/null cannot be opened for a
setuid program.

The fdcheckstd() function makes sure fds 0, 1 and 2 are open by opening
/dev/null. If this fails (e.g. missing devfs or wrong permissions),
fdcheckstd() will return failure and the process will exit as if it received
SIGABRT. The KASSERT is only to check that kern_open() returns the expected
fd, given that it succeeded.

Tripping the KASSERT is most likely if fd 0 is open but fd 1 or 2 are not.