Resolve -Wnull-dereference clang warnings

Clang will simply delete the indirection of the non-volatile null
pointer.
Add -z / --show-ip support

This new option displays both the hostname and the IP address of a
host in the path.  This is important when e.g. the hostname determined
by a reverse lookup has no corresponding forward record.

This is similar to the -b (both) option in tracepath, but -b was
already taken for --bitpattern.

In general we only show the IP address when there is a hostname to
display, but in the split view we add a field consistently for IP
address to the output.