It causes segfault and fixes #34

Passing NULL to strcmp could be unsafe or at the minimum
result in undefined behaviour. g_strcmp0 handles NULL
gracefully and returns 0.

Make sure we don't write past the bounds of the output buffer - thanks
to JeanFI for noticing this issue.

This reverts commit d8579bd621de7940780565343d4d8df7c50f9403.