patch 9.0.1334: using tt_member for the class leads to mistakes

Problem:    Using tt_member for the class leads to mistakes.
Solution:   Add a separate tt_class field.
diff --git a/src/vim9expr.c b/src/vim9expr.c
index 2fa8b57..b8458aa 100644
--- a/src/vim9expr.c
+++ b/src/vim9expr.c
@@ -263,7 +263,7 @@
 	return FAIL;
     }
 
-    class_T *cl = (class_T *)type->tt_member;
+    class_T *cl = type->tt_class;
     int is_super = type->tt_flags & TTFLAG_SUPER;
     if (type == &t_super)
     {