Fail to choose index join in when index has time column.

[winoros]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

create table t(a int, b datetime, index idx(a, b));
create table tt(a int, b datetime);
explain select /*+ TIDB_INLJ(tt) */ * from t join tt where t.a=tt.a and t.b=tt.b;

2. What did you expect to see?

It chooses index join.

3. What did you see instead?

Failed. In log:

[types:1292]invalid time format: '1'

part of error stack:
github.com/pingcap/tidb/types.parseDateTimeFromNum
	/Users/yiding/gocode/src/github.com/pingcap/tidb/types/time.go:1172
github.com/pingcap/tidb/types.ParseTimeFromNum
	/Users/yiding/gocode/src/github.com/pingcap/tidb/types/time.go:1297
github.com/pingcap/tidb/types.(*Datum).convertToMysqlTime
	/Users/yiding/gocode/src/github.com/pingcap/tidb/types/datum.go:992
github.com/pingcap/tidb/types.(*Datum).ConvertTo
	/Users/yiding/gocode/src/github.com/pingcap/tidb/types/datum.go:690
github.com/pingcap/tidb/util/ranger.convertPoint
	/Users/yiding/gocode/src/github.com/pingcap/tidb/util/ranger/ranger.go:93
github.com/pingcap/tidb/util/ranger.appendPoints2IndexRange
	/Users/yiding/gocode/src/github.com/pingcap/tidb/util/ranger/ranger.go:159
github.com/pingcap/tidb/util/ranger.appendPoints2Ranges
	/Users/yiding/gocode/src/github.com/pingcap/tidb/util/ranger/ranger.go:145
github.com/pingcap/tidb/util/ranger.buildCNFIndexRange
	/Users/yiding/gocode/src/github.com/pingcap/tidb/util/ranger/ranger.go:308
github.com/pingcap/tidb/util/ranger.detachCNFCondAndBuildRangeForIndex
	/Users/yiding/gocode/src/github.com/pingcap/tidb/util/ranger/detacher.go:165
github.com/pingcap/tidb/util/ranger.DetachCondAndBuildRangeForIndex
	/Users/yiding/gocode/src/github.com/pingcap/tidb/util/ranger/detacher.go:323
github.com/pingcap/tidb/planner/core.(*LogicalJoin).buildRangeForIndexJoin
	/Users/yiding/gocode/src/github.com/pingcap/tidb/planner/core/exhaust_physical_plans.go:545

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

master 6c242ad