Fix handling of getifaddr return when an interface does not have an address

     Started getting crashes on a linux machine after a network configuration
	 change, this was traced to ImpZ___GetIF_IPAddress() was not correctly
         handling cases where getifaddrs() returns
         items with the if_addr null.  Just continue
         the loop in those cases.  Thanks to Mike Birchall for debugging
	 this one.

         This could happen if the interface doesn't have any addresses, such
         as a tunneling interface (sudo ip tuntap add tun42 mode tun)