Fix uninitialized-variable bug.

map_partition_varattnos() failed to set its found_whole_row output
parameter if the given expression list was NIL.  This seems to be
a pre-existing bug that chanced to be exposed by commit 6f6b99d1.
It might be unreachable in v10, but I have little faith in that
proposition, so back-patch.

Per buildfarm.

src/backend/catalog/partition.c

@@ -916,11 +916,11 @@ map_partition_varattnos(List *expr, int target_varno,
 						Relation partrel, Relation parent,
 						bool *found_whole_row)
 {
-	AttrNumber *part_attnos;
-	bool		my_found_whole_row;
+	bool		my_found_whole_row = false;
 
-	if (expr == NIL)
-		return NIL;
+	if (expr != NIL)
+	{
+		AttrNumber *part_attnos;
 
 		part_attnos = convert_tuples_by_name_map(RelationGetDescr(partrel),
 												 RelationGetDescr(parent),
@@ -931,6 +931,8 @@ map_partition_varattnos(List *expr, int target_varno,
 											RelationGetDescr(parent)->natts,
 											RelationGetForm(partrel)->reltype,
 											&my_found_whole_row);
+	}
+
 	if (found_whole_row)
 		*found_whole_row = my_found_whole_row;