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

src/chain.h

@@ -263,6 +263,7 @@ public:
 
     uint256 GetBlockHash() const
     {
+        assert(phashBlock != nullptr);
         return *phashBlock;
     }
 

src/validation.cpp

@@ -2263,7 +2263,6 @@ bool CChainState::ConnectBlock(const CBlock& block, BlockValidationState& state,
         m_blockman.m_dirty_blockindex.insert(pindex);
     }
 
-    assert(pindex->phashBlock);
     // add this block to the view's block chain
     view.SetBestBlock(pindex->GetBlockHash());