fuzz: Add CScriptNum::getint coverage

Summary: Backport of core PR18510.

Test Plan:

ninja bitcoin-fuzzers
./test/fuzz/test_runner.py -j20 <path_to_corpus> integer scriptnum_ops

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8986