MFC r298368,r298660:

r298368:

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"

CID: 1251410

r298660:

Fix up r298368

Remove the semicolon accidentally added after the new conditional that tests
that /dev/zero is opened successfully.

CID: 1354980