pubkey: Assert CPubKey's ECCVerifyHandle precondition

Summary:

This makes it more clear for fuzzing harness writers and others that ECCVerifyHandle is expected to be held when interacting with CPubKey.

Related PR17274 (D6883).

This is a backport of Core PR17275

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8301