Fix missing null-terminator.

[1uc]

Would have been detectable with ASAN enabled version of NRN.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 85.49%. Comparing base (b4ef537) to head (aea54c0). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1291 +/- ## ========================================== - Coverage 85.50% 85.49% -0.02% ========================================== Files 178 178 Lines 13446 13449 +3 ========================================== + Hits 11497 11498 +1 - Misses 1949 1951 +2 ```

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[bbpbuildbot]

Logfiles from GitLab pipeline #214490 (:white_check_mark:) have been uploaded here!

Status and direct links:

• :white_check_mark: spack_setup
• :white_check_mark: build:intel
• :white_check_mark: build:nvhpc
• :white_check_mark: test:intel
• :white_check_mark: test:nvhpc

[1uc]

This fixes a memory leak? If so, what's a MWE that would demonstrate the problem?

No, it fixes an out-of-bounds access. It's covered many times already; but without an NRN with ASAN enabled (or similar) there's no guarantee that it results in a failure (e.g. SEGFAULT).