CBlockIndex: ensure phashBlock is not nullptr before dereferencing
Summary:
and remove a now-redundant assert preceding a GetBlockHash() caller.
This protects against UB here, and in case of failure (which would
indicate a consensus bug), the debug log will print
bitcoind: blockindex.h:148: BlockHash CBlockIndex::GetBlockHash() const: Assertion `phashBlock != nullptr' failed. Aborted
instead of
Segmentation fault
This is a partial backport of core#25349
https://github.com/bitcoin/bitcoin/pull/25349/commits/14aeece462b149eaf0d28a37d55cc169df99b2cb
Depends on D16069
Test Plan: ninja all check-all
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D16070