Display proper IPv4 address if the correct interface isn't found.

In the impossible case the Clang Static Analyzer had found, we might
even dereference NULL.  That's if an address appeared, was asked for
a UDP socket on it, it arrived and there were no interfaces present.

Trusting "na" is just wrong here, print whatever came from priv child.

Signed-off-by: Martin Pelikan <pelikan@storkhole.cz>