Fixed bugs in BufferedBlockMgr and PA.

In the case of OOM in the BufferedBlockMgr, it was returning a dangling
block pointer instead of NULL.

In PA, when spilling, there was the possibility of forgetting to finalize
the last tuple we tried to add, leading to leaked memory in the UDA.

Change-Id: Id77037aec6b351c19ec8338198a98f0b74d7addc
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4383
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: Nong Li <nong@cloudera.com>