script: prevent UB when computing abs value for num opcode serialize

Summary: Backport of core PR18413.

Test Plan:

ninja all check-all
ninja bitcoin-fuzzers
./test/fuzz/test_runner.py <path_to_corpus>

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9017