[SPARK-3032][Shuffle] Fix key comparison integer overflow introduced sorting exception

Previous key comparison in `ExternalSorter` will get wrong sorting result or exception when key comparison overflows, details can be seen in [SPARK-3032](https://issues.apache.org/jira/browse/SPARK-3032). Here fix this and add a unit test to prove it.
OK, thanks for your advice,  will do it.