updated for version 7.4.609

Problem:    For complicated list and dict use the garbage collector can run
            out of stack space.
Solution:   Use a stack of dicts and lists to be marked, thus making it
            iterative instead of recursive. (Ben Fritz)

src/structs.h

@@ -1223,6 +1223,20 @@ struct dictvar_S
     dict_T	*dv_used_prev;	/* previous dict in used dicts list */
 };
 
+/* structure used for explicit stack while garbage collecting hash tables */
+typedef struct ht_stack_S
+{
+    hashtab_T		*ht;
+    struct ht_stack_S	*prev;
+} ht_stack_T;
+
+/* structure used for explicit stack while garbage collecting lists */
+typedef struct list_stack_S
+{
+    list_T		*list;
+    struct list_stack_S	*prev;
+} list_stack_T;
+
 /* values for b_syn_spell: what to do with toplevel text */
 #define SYNSPL_DEFAULT	0	/* spell check if @Spell not defined */
 #define SYNSPL_TOP	1	/* spell check toplevel text */