Don't remove RM vcore callback if it was never added
Don't remove RM vcore callback if it was never added

It's legal to call Close() without ever calling Open(). The HDFS scan
node only attaches a 'vcore' callback in Open(), but always tries to
remove it in Close() which can hit a DCHECK.

Change-Id: I2481a24cecff9c86083a837e2d1b53eb68df7d81
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4619
Reviewed-by: Alex Behm <fe1626037acfc2dc542d2aa723a6d14f2464a20c@cloudera.com>
Tested-by: jenkins
(cherry picked from commit 0a8ee5448acea0d7fa98b6a17de968d818ee9af2)
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4668
Reviewed-by: Henry Robinson <3eca10f30041813f045165784e24b5a950a6cc7e@cloudera.com>

placrosse/ImpalaToGo,placrosse/ImpalaToGo,ImpalaToGo/ImpalaToGo,ImpalaToGo/ImpalaToGo,ImpalaToGo/ImpalaToGo,ImpalaToGo/ImpalaToGo,placrosse/ImpalaToGo,placrosse/ImpalaToGo,placrosse/ImpalaToGo,placrosse/ImpalaToGo,ImpalaToGo/ImpalaToGo,ImpalaToGo/ImpalaToGo