LibGfx/JPEG: Make sure sampling factors are in the allowed range

[LucasChollet]

As written in the spec (B.2.2 Frame header syntax), only values between 0 and 4 are allowed.

This fixes oss-fuzz issue 68784:
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=68784

[nico]

Is the oss fuzz test small enough to be included as a test? (Not super important.)

[LucasChollet]

Yep, it's not even a kilo. I have a bunch lying around on my computer (mainly for TIFF though), I was starting to think that I should push all of them. Do you think they are all worth their own test, or just one iterating over all of them? IMO the latter is good enough. If you agree, I will then add this PR's test with the others.

[nico]

Single test with loop seems fine 🙂

[stale]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. Thank you for your contributions!

[nico]

If you still have the test files, put them in another PR. I figured merging this without test is better than having stalebot close it.

[LucasChollet]

I figured merging this without test is better than having stalebot close it.

I thought that it was what we agreed on 😅. I was waiting for my two PRs (this one + TIFF) to get merged before adding all my dozen fuzzer-found test cases in one go.

Thanks for the merge!

[nico]

Oooh sorry, I thought you were planning on adding them to the PRs! I'll merge the other one too then.