fix OpenStorageTrie: return an error instead of panicking if the acco…

…unt tree not a verkle tree (#321)

core/state/database.go

@@ -413,7 +413,7 @@ func (db *cachingDB) OpenStorageTrie(stateRoot common.Hash, address common.Addre
 		case *trie.TransitionTrie:
 			return trie.NewTransitionTree(mpt.(*trie.SecureTrie), self.Overlay(), true), nil
 		default:
-			panic("unexpected trie type")
+			return nil, errors.New("expected a verkle account tree, and found another type")
 		}
 	}
 	mpt, err := db.openStorageMPTrie(stateRoot, address, root, nil)

core/state/trie_prefetcher.go

@@ -302,7 +302,7 @@ func (sf *subfetcher) loop() {
 		}
 		sf.trie = trie
 	} else {
-		trie, err := sf.db.OpenStorageTrie(sf.state, sf.addr, sf.root, nil /* safe to set to nil for now, as there is no prefetcher for verkle */)
+		trie, err := sf.db.OpenStorageTrie(sf.state, sf.addr, sf.root, sf.trie)
 		if err != nil {
 			log.Warn("Trie prefetcher failed opening trie", "root", sf.root, "err", err)
 			return