Repair bug #2836: SPI_execute_plan returned zero if none of the querytrees

Repair bug #2836: SPI_execute_plan returned zero if none of the querytrees
were marked canSetTag.  While it's certainly correct to return the result
of the last one that is marked canSetTag, it's less clear what to do when
none of them are.  Since plpgsql will complain if zero is returned, the
8.2.0 behavior isn't good.  I've fixed it to restore the prior behavior of
returning the physically last query's result code when there are no
canSetTag queries.