Fix snprintf buffer length limit

[yuyichao]

The current limit of 17 is wrong since it is not the length of the buffer with the offset j. This causes a hardened build to fail.

[IngoMeyer441]

Thanks for the PR!

Which hardening flags did you use? It would be interesting to add those to our CI debug builds.

[yuyichao]

It was the option that's enabled by default on ArchLinux. I believe it's -D_FORTIFY_SOURCE=3. You can find snprintf in the list at https://www.gnu.org/software/libc/manual/html_node/Source-Fortification.html, among other functions.

[IngoMeyer441]

Ok, great thanks. I already pushed your commit to our CI, it will be automatically mirrored to GitHub in the next minutes. Hopefully, GitHub detects that your commit was already merged in.

[yuyichao]

(And to be clear, if you want to add new options to the CI, by "fail" I meant a runtime failure. The build itself was successful but trying to generate a pdf will fail)

[IngoMeyer441]

Ok, thanks for clarification. I added -D_FORTIFY_SOURCE=3 along with other flags from the OpenSSF recommendations to our Debug CI build. Now, our tests should catch the runtime errors caused by the checks.