IMPALA-4808: old hash join can reference invalid memory
IMPALA-4808: old hash join can reference invalid memory

The bug was that 'probe_rows_exist' could be true even if
there was no current probe row. The node can get into this
state if it takes the branch at line 390.

I tried to reproduce the crash but was unable to after a
few attempts.

Change-Id: Ic068bbc3e029264d1ce814d286e372391639fa68
Reviewed-on: http://gerrit.cloudera.org:8080/5850
Reviewed-by: Matthew Jacobs <df2fa7c39d141dc5dc5589fed74180e757ed6c9a@cloudera.com>
Reviewed-by: Dan Hecht <20ebd99959f0f6e5dcb0fc319b4d8ef53a16b5fb@cloudera.com>
Tested-by: Impala Public Jenkins

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