fix race condition on preimage (#797)

trie/database.go

@@ -722,17 +722,18 @@ func (db *Database) Commit(node common.Hash, report bool, callback func(common.H
  batch := db.diskdb.NewBatch()
 
  // Move all of the accumulated preimages into a write batch
+ db.lock.RLock()
  if db.preimages != nil {
  rawdb.WritePreimages(batch, db.preimages)
  // Since we're going to replay trie node writes into the clean cache, flush out
  // any batched pre-images before continuing.
  if err := batch.Write(); err != nil {
+ db.lock.RUnlock()
  return err
  }
  batch.Reset()
  }
  // Move the trie itself into the batch, flushing if enough data is accumulated
- db.lock.RLock()
  nodes, storage := len(db.dirties), db.dirtiesSize
  db.lock.RUnlock()