addr_resolution_dlg.c: Fix crash when 'Statistics ! Show address resolution' is invoked.

(Introduced when the ipv4_hash_table key format was changed in 5d98ab67)

Change-Id: Ie5d1314f7c7cc8bc71dca8a2db05e34a0a7776d7
Reviewed-on: https://code.wireshark.org/review/3277
Reviewed-by: Bill Meier <wmeier@newsguy.com>