patch 8.2.3809: Vim9: crash when garbage collecting a nested partial

Problem:    Vim9: crash when garbage collecting a nested partial. (Virginia
            Senioria)
Solution:   Set references in all the funcstacks. (closes #9348)
diff --git a/src/structs.h b/src/structs.h
index 5b7739e..9aad415 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -2009,8 +2009,13 @@
  * Structure to hold the context of a compiled function, used by closures
  * defined in that function.
  */
-typedef struct funcstack_S
+typedef struct funcstack_S funcstack_T;
+
+struct funcstack_S
 {
+    funcstack_T *fs_next;	// linked list at "first_funcstack"
+    funcstack_T *fs_prev;
+
     garray_T	fs_ga;		// contains the stack, with:
 				// - arguments
 				// - frame
@@ -2021,7 +2026,7 @@
     int		fs_refcount;	// nr of closures referencing this funcstack
     int		fs_min_refcount; // nr of closures on this funcstack
     int		fs_copyID;	// for garray_T collection
-} funcstack_T;
+};
 
 typedef struct outer_S outer_T;
 struct outer_S {