Properly initialize variables in DecimalAvgGetValue.

This was causing the ASAN failures.

Change-Id: I31b7ea4059d2ac6be295b2f5c19339b79ed3b30e
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4374
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: Nong Li <nong@cloudera.com>