MFC r316993, r316994, r316997 as follows:

r316993:

Fix CID 1372601 in ipfilter/lib/parsefields.c, possible NULL pointer
dereference should reallocarray() fail.

Reported by: Coverity CID 1372601

r316994:

Fix CID 1372600 in ipfilter/tools/ipf_y.y, possible NULL pointer
dereference should reallocarray() fail.

Reported by: Coverity CID 1372600

r316997:

Use warnx() to issue error message.

Reported by: cem