Fix bug in UnwindBlock that could unpark or unfail a parked or failed chain

Summary: Discovered while working on backports

Test Plan:
test_runner.py abc-parkedchain
Fails without the fix and passes with it.

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1982