[SPARK-2412] CoalescedRDD throws exception with certain pref locs

If the first pass of CoalescedRDD does not find the target number of locations AND the second pass finds new locations, an exception is thrown, as "groupHash.get(nxt_replica).get" is not valid.

The fix is just to add an ArrayBuffer to groupHash for that replica if it didn't already exist.

Author: Aaron Davidson <aaron@databricks.com>

Closes #1337 from aarondav/2412 and squashes the following commits:

f587b5d [Aaron Davidson] getOrElseUpdate
3ad8a3c [Aaron Davidson] [SPARK-2412] CoalescedRDD throws exception with certain pref locs
(cherry picked from commit 7c23c0dc3ed721c95690fc49f435d9de6952523c)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>