[Spark-1382] Fix NPE in DStream.slice (updated version of #365)

@zsxwing I cherry-picked your changes and merged the master. #365 had some conflicts once again!

Author: zsxwing <zsxwing@gmail.com>
Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #562 from tdas/SPARK-1382 and squashes the following commits:

e2962c1 [Tathagata Das] Merge remote-tracking branch 'apache-github/master' into SPARK-1382
20968d9 [zsxwing] Replace Exception with SparkException in DStream
e476651 [zsxwing] Merge remote-tracking branch 'origin/master' into SPARK-1382
35ba56a [zsxwing] SPARK-1382: Fix NPE in DStream.slice