Fix assertion in CKey::SignCompact #15299
That's true—however, as long as it has a return value, I think it's correct to check it. It might start to do error handling in the future.
Summary: The second assertion was testing the same condition as the first one. Backport of Core [[bitcoin/bitcoin#15299 | 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