PHJ: Fix block management when spilling.

The previous code did not handle well the case where the spilling happens when
building the hash table (i.e. partitioning the build rows fit). This caused the
probe partition to be starved causing queries that should be able to run to fail
with a not enough buffers error.

Change-Id: I3a9a84e8800a72ed3ce6f5ab7ff03bc2d6eb7ad8
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4403
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: Nong Li <nong@cloudera.com>