patch 9.0.1862: Vim9 Garbage Collection issues

Problem:  Vim9 Garbage Collection issues
Solution: Class members are garbage collected early leading to
          use-after-free problems.  Handle the garbage
          collection of classes properly.

closes: #13019

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>
diff --git a/src/structs.h b/src/structs.h
index 6e9e901..7acd857 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -1525,6 +1525,8 @@
 
     int		class_refcount;
     int		class_copyID;		// used by garbage collection
+    class_T	*class_next_used;	// for list headed by "first_class"
+    class_T	*class_prev_used;	// for list headed by "first_class"
 
     class_T	*class_extends;		// parent class or NULL