Don't cache existing IPv4 and IPv6 hostnames.

Don't cache existing IPv4 and IPv6 hostnames.

In the Qt UI redrawing the packet list redissects our visible packets,
which might call add_ipv{4,6}_name, which sets new_resolved_objects =
TRUE, which emits the addressResolutionChanged signal, which redraws the
packet list, which ...

Break out of the loop by checking to see if we've already cached an IPv4
or IPv6 hostname.

Change-Id: Icb2841e3453fb98d4cf0ea06a0f49737e2b8f25e
Reviewed-on: https://code.wireshark.org/review/7738
Reviewed-by: Gerald Combs <gerald@wireshark.org>