Fix crash when columns are dropped between compute stats calls

FinalizePartitionedColumnStats() should have iterated over the list of
columns present in the table, rather than the list in the existing stats
data structure. If a column was dropped, but still persisted in the old
structure, it was possible that we could index off the end of an array.

Change-Id: Ib1ab7690ffae05afff826b9d1a15871337691739
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5437
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins