HBASE-10957 HMaster can abort with NPE in #rebuildUserRegions (Nicolas Liochon)

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/hbase-10070@1590184 13f79535-47bb-0310-9956-ffa450edef68
Merge branch 'hbase-10070' of github.com:apache/hbase into hbase-10070