functions form as char[N], if execute in dble without [N] section will get error

[irene-coming]

• dble version：
  5.6.29-dble-9.9.9.9-d8a5c79cc4efda58659d7990340fde23777a1558-20190903023556

• preconditions ：
  no

• configs：

schema.xml

rule.xml

server.xml

• steps：
  step1. query to dble:

mysql> select concat(a.name, cast(b.pad as char)) from sharding_3_t1 a, sharding_2_t1 b where a.id = b.id;
ERROR 1003 (HY000): join thread error, String index out of range: -1

• expect result：
  1. step1, query success
• real result：
  1. query failed
• supplements：
  1. other similar functions may also have this problem, eg: nchar, need check
  2. dble.log related info:

2019-09-03 05:38:45.576  INFO [complexQueryExecutor0] (com.actiontech.dble.backend.mysql.nio.handler.query.impl.join.JoinHandler.ownThreadJob(JoinHandler.java:235)) - join thread error, String index out of range: -1
java.lang.StringIndexOutOfBoundsException: String index out of range: -1
        at java.lang.String.substring(String.java:1967) ~[?:1.8.0_121]
        at com.actiontech.dble.plan.common.item.function.castfunc.ItemCharTypeCast.valStr(ItemCharTypeCast.java:54) ~[dble-9.9.9.9.jar:?]
        at com.actiontech.dble.plan.common.item.function.strfunc.ItemStrFunc.valInt(ItemStrFunc.java:66) ~[dble-9.9.9.9.jar:?]
        at com.actiontech.dble.plan.common.item.Item.updateNullValue(Item.java:467) ~[dble-9.9.9.9.jar:?]
        at com.actiontech.dble.plan.common.item.function.ItemFunc.isNull(ItemFunc.java:128) ~[dble-9.9.9.9.jar:?]
        at com.actiontech.dble.plan.common.item.function.strfunc.ItemFuncConcat.valStr(ItemFuncConcat.java:29) ~[dble-9.9.9.9.jar:?]
        at com.actiontech.dble.plan.common.item.Item.getRowPacketByte(Item.java:320) ~[dble-9.9.9.9.jar:?]
        at com.actiontech.dble.plan.common.item.ItemRef.getRowPacketByte(ItemRef.java:123) ~[dble-9.9.9.9.jar:?]
        at com.actiontech.dble.backend.mysql.nio.handler.query.impl.SendMakeHandler.rowResponse(SendMakeHandler.java:100) ~[dble-9.9.9.9.jar:?]
        at com.actiontech.dble.backend.mysql.nio.handler.query.impl.join.JoinHandler.connectLeftAndRight(JoinHandler.java:289) ~[dble-9.9.9.9.jar:?]
        at com.actiontech.dble.backend.mysql.nio.handler.query.impl.join.JoinHandler.ownThreadJob(JoinHandler.java:215) [dble-9.9.9.9.jar:?]
        at com.actiontech.dble.backend.mysql.nio.handler.query.OwnThreadDMLHandler$1.run(OwnThreadDMLHandler.java:64) [dble-9.9.9.9.jar:?]
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) [?:1.8.0_121]
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) [?:1.8.0_121]
        at java.lang.Thread.run(Thread.java:745) [?:1.8.0_121]

[wjl1619]

verified on version: 5.6.29-dble-9.9.9.9-97b9e4cee24afeae57e7890aa5ebba36afcd225b-20191104052741