This allows removing the weird pfree() of the resultTupleSlot's tuple
descriptor. What would've happened without the pfree() is that the old
slot was allocated in the first ExecAssignScanProjectionInfo() call, in
partitionMemoryContext, and then immediately destroyed when the memory
context was reset. The second call to ExecAssignScanProjectionInfo() tries
to free the slot, again, causing the segfault. But we can avoid that
by this rearrangement of the calls in a cleaner way.

In the passing, clean up the code a bit. I found having separate variables,
indexState and scanState, which point to the same struct, to be confusing.