Fix possible infinite loop with Parallel Append.

Fix possible infinite loop with Parallel Append.

When the previously-chosen plan was non-partial, all pa_finished
flags for partial plans are now set, and pa_next_plan has not yet
been set to INVALID_SUBPLAN_INDEX, the previous code could go into
an infinite loop.

Report by Rajkumar Raghuwanshi.  Patch by Amit Khandekar and me.
Review by Kyotaro Horiguchi.

Discussion: http://postgr.es/m/CAJ3gD9cf43z78qY=U=H0HvOEN341qfRO-vLpnKPSviHeWgJQ5w@mail.gmail.com