Assert consistency of currPage that ended scan.

When _bt_readnextpage is called with our nbtree parallel scan already
seized (i.e. when it is directly called by _bt_first), we never expect a
prior call to _bt_readpage for lastcurrblkno to already indicate that
the scan should end -- the _bt_first caller's blkno must always be read.
After all, the "prior" _bt_readpage call (the call for lastcurrblkno)
probably took place in some other backend (and it might not even have
finished by the time our backend reaches _bt_first/_bt_readnextpage).

Add a documenting assertion to the path where _bt_readnextpage ends the
parallel scan based on information about lastcurrblkno from so->currPos.
Assert that the most recent _bt_readpage call that set so->currPos is in
fact lastcurrblkno's _bt_readpage call.

Follow-up to bugfix commit b5ee4e52.

src/backend/access/nbtree/nbtsearch.c

@@ -2230,8 +2230,9 @@ _bt_readnextpage(IndexScanDesc scan, BlockNumber blkno,
 			 !so->currPos.moreRight : !so->currPos.moreLeft))
 		{
 			/* most recent _bt_readpage call (for lastcurrblkno) ended scan */
+			Assert(so->currPos.currPage == lastcurrblkno && !seized);
 			BTScanPosInvalidate(so->currPos);
-			_bt_parallel_done(scan);
+			_bt_parallel_done(scan);	/* iff !so->needPrimScan */
 			return false;
 		}