Replace pindex pointer with block reference

pindex can not be nullptr, so document that, and clear it up in the next
commit.

Author: MarcoFalke

src/node/blockstorage.cpp

@@ -722,29 +722,30 @@ static bool WriteBlockToDisk(const CBlock& block, FlatFilePos& pos, const CMessa
 
 bool BlockManager::WriteUndoDataForBlock(const CBlockUndo& blockundo, BlockValidationState& state, CBlockIndex* pindex, const CChainParams& chainparams)
 {
+    auto& block{*pindex};
     AssertLockHeld(::cs_main);
     // Write undo information to disk
-    if (pindex->GetUndoPos().IsNull()) {
+    if (block.GetUndoPos().IsNull()) {
         FlatFilePos _pos;
-        if (!FindUndoPos(state, pindex->nFile, _pos, ::GetSerializeSize(blockundo, CLIENT_VERSION) + 40)) {
+        if (!FindUndoPos(state, block.nFile, _pos, ::GetSerializeSize(blockundo, CLIENT_VERSION) + 40)) {
             return error("ConnectBlock(): FindUndoPos failed");
         }
-        if (!UndoWriteToDisk(blockundo, _pos, pindex->pprev->GetBlockHash(), GetParams().MessageStart())) {
+        if (!UndoWriteToDisk(blockundo, _pos, block.pprev->GetBlockHash(), GetParams().MessageStart())) {
             return AbortNode(state, "Failed to write undo data");
         }
         // rev files are written in block height order, whereas blk files are written as blocks come in (often out of order)
         // we want to flush the rev (undo) file once we've written the last block, which is indicated by the last height
         // in the block file info as below; note that this does not catch the case where the undo writes are keeping up
         // with the block writes (usually when a synced up node is getting newly mined blocks) -- this case is caught in
         // the FindBlockPos function
-        if (_pos.nFile < m_last_blockfile && static_cast<uint32_t>(pindex->nHeight) == m_blockfile_info[_pos.nFile].nHeightLast) {
+        if (_pos.nFile < m_last_blockfile && static_cast<uint32_t>(block.nHeight) == m_blockfile_info[_pos.nFile].nHeightLast) {
             FlushUndoFile(_pos.nFile, true);
         }
 
         // update nUndoPos in block index
-        pindex->nUndoPos = _pos.nPos;
-        pindex->nStatus |= BLOCK_HAVE_UNDO;
-        m_dirty_blockindex.insert(pindex);
+        block.nUndoPos = _pos.nPos;
+        block.nStatus |= BLOCK_HAVE_UNDO;
+        m_dirty_blockindex.insert(&block);
     }
 
     return true;