Fix index bug in FFTPoissonSolverPeriodic

[AlexanderSinn]

In the loop over m_tmpSpectralField and m_inv_k2, the wrong box was used which was 2x too large, causing memory errors.

• [ ] Small enough (< few 100s of lines), otherwise it should probably be split into smaller PRs
• [ ] Tested (describe the tests in the PR description)
• [ ] Runs on GPU (basic: the code compiles and run well with the new module)
• [ ] Contains an automated test (checksum and/or comparison with theory)
• [ ] Documented: all elements (classes and their members, functions, namespaces, etc.) are documented
• [ ] Constified (All that can be const is const)
• [ ] Code is clean (no unwanted comments, )
• [ ] Style and code conventions are respected at the bottom of https://github.com/Hi-PACE/hipace
• [ ] Proper label and GitHub project, if applicable