IMPALA-1268: Fix initialization/closing of agg_fn_ctxs_ in partitioned agg

I tested this locally, I don't think it's worth adding a regular test
case.

Change-Id: Ifaeb7b5d37bc7c00efe8292cef638beae832b131
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4462
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins