Fix possible crash with Parallel Bitmap Heap Scan.

If a Parallel Bitmap Heap scan's chain of leftmost descendents
includes a BitmapOr whose first child is a BitmapAnd, the prior coding
would mistakenly create a non-shared TIDBitmap and then try to perform
shared iteration.

Report by Tomas Vondra.  Patch by Dilip Kumar.

Discussion: http://postgr.es/m/50e89684-8ad9-dead-8767-c9545bafd3b6@2ndquadrant.com

src/backend/optimizer/plan/createplan.c

@@ -4933,7 +4933,11 @@ bitmap_subplan_mark_shared(Plan *plan)
 		bitmap_subplan_mark_shared(
 								   linitial(((BitmapAnd *) plan)->bitmapplans));
 	else if (IsA(plan, BitmapOr))
+	{
 		((BitmapOr *) plan)->isshared = true;
+		bitmap_subplan_mark_shared(
+								   linitial(((BitmapOr *) plan)->bitmapplans));
+	}
 	else if (IsA(plan, BitmapIndexScan))
 		((BitmapIndexScan *) plan)->isshared = true;
 	else