Fix possible division by zero when using -verbose

[oriyadid]

I encountered an issue where using -verbose on a pure cpp2 file causes a floating point exception, as there are zero cpp1 lines, this PR fixes it.

To reproduce you may compile this simple file (or any other pure cpp2 file) with -verbose, other flags don't matter:

main: () -> int = {
  return 0;
}

[hsutter]

Thanks for your pull request! It looks like this may be your first contribution to cppfront. I've sent the Contributor License Agreement (CLA) to your email, and once it's signed I can look at your pull request. Thanks again for your contribution.

[oriyadid]

I've now signed the CLA, thanks.

[hsutter]

Thanks!