Release build testing results:
- GZDoom g4.3pre-389-gb9367caa6 (includes phantombeta's fix) was run 2329 times with a test version of my mod. No errors of any kind were encountered.
- For comparison, I also tested GZDoom g4.3pre-388-ga07d7856c, which does not have the fix. Of the 2267 test runs, there were 5 invalid instruction errors and 3 write to address zero errors.
So, regardless of what Nash says, I'd say this is fixed... at least for me. Maybe Nash's project has something that triggers a bug in some other place of GZDoom source (like phantombeta said, could be the asmjit library).
It is also interesting that all address zero errors seemed to have the same stack trace this time. Unfortunately, I couldn't get the exact stack trace again - I was running GZDoom with +logfile, but none of the errors appeared in my logs (this may be yet another bug). I only know the errors happened because I set my script to take a screenshot of GZDoom's window in case of a crash (I couldn't leave GZDoom windows open because my laptop could've run out of memory).