fuzz: fix unused variable compiler warning

Summary:
Backport of core PR18664.

The warning was previously fixed in D8908 but this was invalidating our
seeds.

Test Plan:

ninja bitcoin-fuzzers

Ensure there is no warning.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9004