[refactor multisig] separate nullfail from stack cleanup

Summary:
Currently, some rule checks are integrated into the stack cleanup
process, resulting in some obfuscated-looking behaviour.

This moves the stack cleanup to the very end, once all operations
have completed, including nullfail and the dummy element check.

Depends on D3673

Test Plan: make check

Reviewers: deadalnix, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3626