test: fix `platform_output` netmask printing.
test: fix `platform_output` netmask printing.

The implementation will leave the family set to `AF_UNSPEC` if a
netmask is not present, but the test driver would always print the
uninitialized buffer as an `AF_INET4` address.  It will now print
"none" if there is no netmask (e.g., for loopback interfaces).

PR-URL: https://github.com/libuv/libuv/pull/373
Reviewed-By: Ben Noordhuis <59bd0a3ff43b32849b319e645d4798d8a5d1e889@bnoordhuis.nl>
Reviewed-By: Saúl Ibarra Corretgé <8a7991f6950ea189abf159d6ec98d2fcc13dc5f6@gmail.com>

unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso