- Fix dnstap test program, cleans up to have clean memory on exit,

- Fix dnstap test program, cleans up to have clean memory on exit,
  for tap_data_free, does not delete NULL items. Also it does not try
  to free the tail, specifically in the free of the list since that
  picked up the next item in the list for its loop causing invalid
  free. Added internal unit test to unbound-dnstap-socket for that.