Add guard code to prevent the use of NULL pointers in emitter.c

[rsbeckerca]

This is related to CVE-2024-35326

Fixes #301

[rsbeckerca]

This is a candidate contribution of defensive guard code that I added in my own fork to deal with potential NULL pointers and double-frees in the emitter.c code. If this is useful, please consider it.

[rsbeckerca]

Looking at the CI builds, I don't think the failures are related to this contribution, but I might be wrong about that.

[perlpunk]

Thanks! I had also been playing around with setting things to NULL after freeing, but ran into problems.
But I think the test failure might be related:
https://github.com/yaml/libyaml/actions/runs/10301257382/job/28516105493?pr=305#step:9:189

[perlpunk]

btw, the linked CVE is about heap-buffer-overflow, not double free

[rsbeckerca]

I'm not sure I understand the test failure. Is this because if a NULL is passed in, it previously ignored it? Or is that a free after free not working anymore - because that would be an actual bug. I did not change that much.

[rsbeckerca]

@perlpunk Is there a command I can run to test the CI failure? Having trouble viewing it. I would like to make this work.