core: skip checking state root existence when do snapsync by fast node (ethereum#2258)

Author: buddh0

core/blockchain.go

@@ -406,7 +406,7 @@ func NewBlockChain(db ethdb.Database, cacheConfig *CacheConfig, genesis *Genesis
 	// Make sure the state associated with the block is available, or log out
 	// if there is no available state, waiting for state sync.
 	head := bc.CurrentBlock()
-	if !bc.HasState(head.Root) {
+	if !bc.NoTries() && !bc.HasState(head.Root) {
 		if head.Number.Uint64() == 0 {
 			// The genesis state is missing, which is only possible in the path-based
 			// scheme. This situation occurs when the initial state sync is not finished
@@ -1011,7 +1011,7 @@ func (bc *BlockChain) SnapSyncCommitHead(hash common.Hash) error {
 			return err
 		}
 	}
-	if !bc.HasState(root) {
+	if !bc.NoTries() && !bc.HasState(root) {
 		return fmt.Errorf("non existent state [%x..]", root[:4])
 	}
 	// If all checks out, manually set the head block.