Update amount's modulo to be the proper type.

Summary:
Computer define division and modulo such as

b * (a / b) + a % b = a

It follows that a % b is the same type as a.

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1702