IMPALA-549: Failure during Prepare() of TopNNode causes segfault due to NULL ptr access
IMPALA-549: Failure during Prepare() of TopNNode causes segfault due to NULL ptr access

Change-Id: I5756195dcc818db0cbd8a28434363dae93aa77a5
Reviewed-on: http://gerrit.ent.cloudera.com:8080/296
Tested-by: jenkins
Reviewed-by: Henry Robinson <3eca10f30041813f045165784e24b5a950a6cc7e@cloudera.com>

michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala,michaelhkw/incubator-impala,cloudera/Impala,michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala,michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala