Seekable format empty string

[daniellerozenblit]

This PR is a followup to a previous PR by @yhoogstrate. The goal of the previous PR was to fix a small bug in seekable_format that led to the omission of the ZSTD MAGIC when compressing an empty string.

I have validated the bug fix with the test case provided, but have created this PR to fix some minor issues (forgetting to free malloc'd memory in the test case).

[terrelln]

This looks good to me once CI passes!

Only question is do you know if the seekable format tests run in CI?

[Cyan4973]

Only question is do you know if the seekable format tests run in CI?

Good point,
Seekable format tests can be triggered directly,
or as part of contrib/ tests,
which itself can also be triggered as part make test-all.

But ... looking at the list of tests on Github Actions,
it's unclear to me if any of these tests are run.
I'm sure they used to be part of CI,
but it might have been in the TravisCI era,
and they could have been forgotten in the transition towards Github Actions ...

edit : I can't find any trace of contrib/ tests within .github/ workflows,
so we actually might have to add this test.

edit 2: Actually, make contrib is also part of make all,
and we do have one make all test,
and it runs the seekable format tests :
https://github.com/facebook/zstd/actions/runs/3688718449/jobs/6243837843#step:3:228

So that's fine !

[yhoogstrate]

wow thanks @daniellerozenblit to carry this forwards ! Good to see the test works and the contrib tests are included in the CI. Sorry I missed the free after malloc 😳