[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.
strictly stylistic -- it made more sense when I was using put below, now there's no reason for it