[SPARK-17685][SQL] Make SortMergeJoinExec's currentVars is null when calling createJoinKey

## What changes were proposed in this pull request?

The following SQL query cause `IndexOutOfBoundsException` issue when `LIMIT > 1310720`:
```sql
CREATE TABLE tab1(int int, int2 int, str string);
CREATE TABLE tab2(int int, int2 int, str string);
INSERT INTO tab1 values(1,1,'str');
INSERT INTO tab1 values(2,2,'str');
INSERT INTO tab2 values(1,1,'str');
INSERT INTO tab2 values(2,3,'str');

SELECT
  count(*)
FROM
  (
    SELECT t1.int, t2.int2
    FROM (SELECT * FROM tab1 LIMIT 1310721) t1
    INNER JOIN (SELECT * FROM tab2 LIMIT 1310721) t2
    ON (t1.int = t2.int AND t1.int2 = t2.int2)
  ) t;
```

This pull request fix this issue.

## How was this patch tested?

unit tests

Author: Yuming Wang <wgyumg@gmail.com>

Closes #17920 from wangyum/SPARK-17685.