IMPALA-959: Fix ASAN decimal crashes.

Not quite sure what the underlying issue is but these fixes seem to work.

Change-Id: I759804eb8338ba86969c0214a1e6e35588c94297
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2698
Reviewed-by: Marcel Kornacker <marcel@cloudera.com>
Tested-by: jenkins