Use size_t for stack index in OP_MULTISIG

Summary: Indices are used to index into the stack and to be compared with the stack size. Even though we don't expect the stack to a billions of elements big, it is correct to use size_t here.

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Subscribers: markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3648