Fix assertion in CKey::SignCompact
Summary:
The second assertion was testing the same condition as the first one.
Backport of Core PR15299
Test Plan: ninja && ninja check-all
Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien, deadalnix
Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien, deadalnix
Subscribers: Fabien
Differential Revision: https://reviews.bitcoinabc.org/D7704