[master] Fix bug in TableServers.prefetchRegionCache that fails TestHTable

Author: daviddeng

Summary: In `getCachedHRegionLocations` a `null` is set to `row` because it doesn't care about the result. So add a check in `prefetchRegionCache`.

Test Plan: `TestHTable`

Reviewers: manukranthk, liyintang, fan, gauravm

Reviewed By: fan

CC: hbase-eng@

Differential Revision: https://phabricator.fb.com/D1247604

Task ID: 3296253

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.89-fb@1584187 13f79535-47bb-0310-9956-ffa450edef68