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 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