Fix coverity issue with contrib/netbsd-tests/lib/libc/ssp/h_read.c

Ensure opening /dev/zero succeeds. Abort the test if it doesn't.

Also, use _PATH_DEVZERO instead of hardcoding "/dev/zero"

MFC after: 2 weeks
CID: 1251410
Reported by: Coverity
Sponsored by: EMC / Isilon Storage Division"