[Rodrigodd]

Problem: fstGetSVarint64() accumulates the varint in an int64_t, so a long enough varint shifts past the width of the type, which is undefined behaviour and is reported by UBSan.

Solution: accumulate in a uint64_t, dropping the bits that do not fit.

[Rodrigodd]

CI failed on Windows, but it was in a pre-existing test. I think it is unrelated to the changes here and is probably due to some race/timing issue.

An AI review pointed out that tmpfile_open on Windows uses GetTempFileName with uUnique set to 0, which derives the filename from the current system time. This means that multiple tests running in parallel are very likely to cause problems.

I will create another branch with a fix for that. Rerunning the CI here should make it pass.

Edit: actually GetTempFileName guarantees that the filename is unique while the file exists, but that function opens the file with unlink_fopen which deletes and recreate the file non-atomically, creating a gap where another process could open the same file.

[tbybell]

OK, #30 passes checks now, thanks.