Pull minimal check out of CScriptNum constr. into IsMinimalArray

Summary:
Move the check for minimal encoded arrays out of CScriptNum's
constructor and into a free function. This enables it to be used
elsewhere, and avoid hitting exceptions.

Test Plan: make check

Reviewers: movrcx, jasonbcox, matiu, deadalnix, #bitcoin_abc

Reviewed By: jasonbcox, #bitcoin_abc

Subscribers: shadders, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1214