[sync] fix testnet syncing panic issue (#3308)

* [sync] fix testnet syncing panic issue

* [sync] add back close connections in initialization

api/service/syncing/syncing.go

@@ -102,6 +102,7 @@ func CreateStateSync(ip string, port string, peerHash [20]byte) *StateSync {
 	stateSync.selfPeerHash = peerHash
 	stateSync.commonBlocks = make(map[int]*types.Block)
 	stateSync.lastMileBlocks = []*types.Block{}
+	stateSync.syncConfig = &SyncConfig{}
 	return stateSync
 }