libyaml: Fix yaml_write_handler return values

[perlpunk]

See #11811

libyaml expects 1 for success and 0 for failure.

https://github.com/yaml/libyaml/blob/master/src/writer.c#L53-L62

if (emitter->write_handler(emitter->write_handler_data, ...) {
    ...
}
else {
    return yaml_emitter_set_writer_error(emitter, "write error");
}

The logic is currently reverse, which (in combination of the failing check for the yaml_emitter_dump return value) caused several wrong bug reports and a CVE.

The fuzzer programs just ignore the failing yaml_emitter_dump, and so I assume it never appeared as a problem. Only in the cases where the wrongly called yaml_emitter_close ran into a case where it popped from an empty stack an overflow was detected.

The input YAML in question just had a lot of nested sequences in the form

- - - - -

which in canonical output mode resulted in a large output because of the indentation, and so the buffer flush was triggered before the emitter finished:

!!seq [
  !!seq [
    ...

In the most cases the YAML is simply too small to produce the error because the flush happened when the output was complete.

Note that this does not yet fix the missing error handling of yaml_emitter_dump in libyaml_dumper_fuzzer.c etc.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[github-actions]

perlpunk is a new contributor to projects/libyaml. The PR must be approved by known contributors before it can be merged. The past contributors are: ingydotnet, alex, rjotwani, devtty1er, Dor1s, ssbr (unverified), sigmavirus24 (unverified), inferno-chromium (unverified), mikea (unverified)