Fix QHostInfo IP resolution when there's no reverse for the IP.
If you try to resolve 10.3.4.6, you're probably going to get that it
doesn't exist. On some systems, getnameinfo will return the IP address
in string form (Linux, without NI_NAMEREQD). On some others, it will
fail (Mac, Windows).
So harmonise by gracefully handling the case in which getnameinfo
fails.
Possible behaviour change: we don't try the forward resolution any
more, after completing the reverse one.
Reviewed-by: Peter Hartmann <peter.hartmann@nokia.com>