Lost connection to tidb-server when query a wrong format TIME

[iosmanthus]

Bug Report

1. What did you do?
   select time("- -")
select time("---1)
select time("-- --1)

2. What did you expect to see?
   report errors correctly, instead of lost connection to tidb-server.

3. What did you see instead?

> select time("- -")
Reconnecting...
Reconnecting...

And here is the log of tidb-server when I trigger the bug:

[2019/04/29 18:51:56.421 +08:00] [INFO] [server.go:361] ["new connection"] [conn=1] [remoteAddr=127.0.0.1:52014]
[2019/04/29 18:51:56.624 +08:00] [INFO] [server.go:361] ["new connection"] [conn=2] [remoteAddr=127.0.0.1:52018]
[2019/04/29 18:51:57.629 +08:00] [INFO] [server.go:363] ["close connection"] [conn=2]
[2019/04/29 18:52:01.997 +08:00] [INFO] [server.go:363] ["close connection"] [conn=1]
[2019/04/29 18:52:03.200 +08:00] [INFO] [server.go:361] ["new connection"] [conn=3] [remoteAddr=127.0.0.1:52022]
[2019/04/29 18:52:03.202 +08:00] [INFO] [server.go:363] ["close connection"] [conn=3]
[2019/04/29 18:52:04.219 +08:00] [INFO] [server.go:361] ["new connection"] [conn=4] [remoteAddr=127.0.0.1:52026]

4. What version of TiDB are you using?

Release Version: v2.1.8
Git Commit Hash: 9a2d2da372947a50a02f9b9238a49f2db7ab9971
Git Branch: HEAD
UTC Build Time: 2019-04-12 07:48:47
GoVersion: go version go1.11.2 linux/amd64
Race Enabled: false
TiKV Min Version: 2.1.0-alpha.1-ff3dd160846b7d1aed9079c389fc188f7f5ea13e
Check Table Before Drop: false

[winkyao]

hum, looks like a bug, we will look into this.

[zz-jason]

@erjiaqing @yangxuanjia PTAL

[winkyao]

@iosmanthus I reproduce this issue in the master branch.