Error handler segfault

[kustra]

This line results in a segfault in case of an error if I don't have an error handler set up:

mpack/src/mpack/mpack-writer.c

Line 1769 in df5860e

writer->error_fn(writer, writer->error);

I see that error_fn is checked for NULL everywhere else before call.

Nice work, btw. I like the API design.

[ludocode]

Fixed! Sorry about this. I need to plan a release soon to get a few of these builder bugs out.