CBlockIndex: ensure phashBlock is not nullptr before dereferencing

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: chain.h:265: uint256 CBlockIndex::GetBlockHash() const: Assertion `phashBlock != nullptr' failed.
Aborted

instead of

Segmentation fault

1 files changed, 1 insertions, 0 deletions

diff --git a/src/chain.h b/src/chain.h
index ecc2ae732f..1a499b07cc 100644
--- a/src/chain.h
+++ b/src/chain.h
@@ -263,6 +263,7 @@ public:
 
     uint256 GetBlockHash() const
     {
+        assert(phashBlock != nullptr);
         return *phashBlock;
     }