MDEV-472 mysql-test-run --valgrind main.ps_2myisam gives warning about not initialized memory

Item::get_date() should return 1 unless the value is a valid date.

sql/item_subselect.cc

@@ -1218,7 +1218,7 @@ bool Item_singlerow_subselect::get_date(MYSQL_TIME *ltime,uint fuzzydate)
   else
   {
     reset();
-    return 0;
+    return 1;
   }
 }