Show better error message, when the src/dst offset is negative

[gzsombor]

No description provided.

[luben]

Hi, thanks for your contribution. Unfortunately I see a problem here: you modify src/main/native/common/error_private.c that comes from upstream (https://github.com/facebook/zstd). I would prefer not to do any changes on these files as they will incur maintenance overhead on each new upstream version - I have not to forget to apply these changes. Also the private files don't have any stability guarantee so they may go away or get refactored making similar change in the future more complicated

[gzsombor]

Thanks for the explanation, that's a reasonable concern. I guess, src/main/native/zstd_errors.h is also coming from upstream, so you would rather not touch that too, am I right?
I can imagine, creating separate error constants in jni_fast_zstd.c and put the error message somewhere there could work, but I'm afraid that it will be a bit more complicated. Would you merge a solution like that, or any idea, how to solve this problem?

[luben]

Right, native/zstd_errors.h also comes from upstream. I also don't think that maintaining our own message types in jni_fast_zstd is good idea.

I think for that case, may be better approach is to check for negative offsets on the Java side, before calling the JNI code and raising an exception from there.