Fix crash in assign_collations_walker for EXISTS with empty SELECT list.

We (I think I, actually) forgot about this corner case while coding
collation resolution.  Per bug #8648 from Arjen Nienhuis.

src/backend/parser/parse_collate.c

@@ -484,16 +484,22 @@ assign_collations_walker(Node *node, assign_collations_context *context)
 				 * SubLink.  Act as though the Query returns its first output
 				 * column, which indeed is what it does for EXPR_SUBLINK and
 				 * ARRAY_SUBLINK cases.  In the cases where the SubLink
-				 * returns boolean, this info will be ignored.
+				 * returns boolean, this info will be ignored.	Special case:
+				 * in EXISTS, the Query might return no columns, in which case
+				 * we need do nothing.
 				 *
 				 * We needn't recurse, since the Query is already processed.
 				 */
 				Query	   *qtree = (Query *) node;
 				TargetEntry *tent;
 
+				if (qtree->targetList == NIL)
+					return false;
 				tent = (TargetEntry *) linitial(qtree->targetList);
 				Assert(IsA(tent, TargetEntry));
-				Assert(!tent->resjunk);
+				if (tent->resjunk)
+					return false;
+
 				collation = exprCollation((Node *) tent->expr);
 				/* collation doesn't change if it's converted to array */
 				strength = COLLATE_IMPLICIT;

src/test/regress/expected/subselect.out

@@ -700,3 +700,13 @@ explain (verbose, costs off)
                  One-Time Filter: ("*VALUES*".column1 = "*VALUES*".column1)
 (8 rows)
 
+--
+-- Check we behave sanely in corner case of empty SELECT list (bug #8648)
+--
+create temp table nocolumns();
+select exists(select * from nocolumns);
+ exists 
+--------
+ f
+(1 row)
+

src/test/regress/sql/subselect.sql

@@ -405,3 +405,9 @@ explain (verbose, costs off)
 explain (verbose, costs off)
   select x, x from
     (select (select random() where y=y) as x from (values(1),(2)) v(y)) ss;
+
+--
+-- Check we behave sanely in corner case of empty SELECT list (bug #8648)
+--
+create temp table nocolumns();
+select exists(select * from nocolumns);