Fix ubsan failure in excessiveblock_tests
Fix ubsan failure in excessiveblock_tests

Summary: Introduced in D5136.

Test Plan: Run the UBSAN build.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5142

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