[LINTER] Work around a cppcheck false positive

Summary:
Cppcheck is raising another new false positive container out of bounds in the processor tests. Fortunately this time we can work around it easily without having to add an exception.
Note: emplace() would be better but cppcheck disagrees.

Test Plan:

ninja check-avalanche
arc lint

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9765