fuzz: fix vector size problem in system fuzzer
fuzz: fix vector size problem in system fuzzer

Summary:
```
This PR fixes a problem with vector resizing in system fuzzer (case 7
there). Originally, this problem was discussed in PR #18908
```

Backport of core [[https://github.com/bitcoin/bitcoin/pull/18917 | PR18917]].

Test Plan:
  ninja bitcoin-fuzzers
  ./src/test/fuzz/system <path_to_corpus>

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9059

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