Ensure the view's best Orchard anchor matches the previous block

`ConnectBlock` was already checking that the given view's "best block"
was the previous block. However, it then assumed the view was correct on
its claimed best anchors.

For Orchard, we know that `hashFinalOrchardRoot` field of `CBlockIndex`
will only ever be set when a block is (attempted to be) connected to the
main chain, and so we can instead add assertions around its value and
ensure the view is consistent with the previous block.

src/main.cpp

@@ -3136,7 +3136,19 @@ bool ConnectBlock(const CBlock& block, CValidationState& state, CBlockIndex* pin
     assert(view.GetSaplingAnchorAt(view.GetBestAnchor(SAPLING), sapling_tree));
 
     OrchardMerkleFrontier orchard_tree;
-    assert(view.GetOrchardAnchorAt(view.GetBestAnchor(ORCHARD), orchard_tree));
+    if (pindex->pprev && chainparams.GetConsensus().NetworkUpgradeActive(pindex->pprev->nHeight, Consensus::UPGRADE_NU5)) {
+        // Verify that the view's current state corresponds to the previous block.
+        assert(pindex->pprev->hashFinalOrchardRoot == view.GetBestAnchor(ORCHARD));
+        // We only call ConnectBlock() on top of the active chain's tip.
+        assert(!pindex->pprev->hashFinalOrchardRoot.IsNull());
+
+        assert(view.GetOrchardAnchorAt(pindex->pprev->hashFinalOrchardRoot, orchard_tree));
+    } else {
+        if (pindex->pprev) {
+            assert(pindex->pprev->hashFinalOrchardRoot.IsNull());
+        }
+        assert(view.GetOrchardAnchorAt(OrchardMerkleFrontier::empty_root(), orchard_tree));
+    }
 
     // Grab the consensus branch ID for this block and its parent
     auto consensusBranchId = CurrentEpochBranchId(pindex->nHeight, chainparams.GetConsensus());