patch 9.0.0949: crash when unletting a variable while listing variables

Problem:    Crash when unletting a variable while listing variables.
Solution:   Disallow changing a hashtable while going over the entries.
            (closes #11435)
diff --git a/src/proto/dict.pro b/src/proto/dict.pro
index a4442cc..fdccca5 100644
--- a/src/proto/dict.pro
+++ b/src/proto/dict.pro
@@ -10,7 +10,7 @@
 int dict_free_nonref(int copyID);
 void dict_free_items(int copyID);
 dictitem_T *dictitem_alloc(char_u *key);
-void dictitem_remove(dict_T *dict, dictitem_T *item);
+void dictitem_remove(dict_T *dict, dictitem_T *item, char *command);
 void dictitem_free(dictitem_T *item);
 dict_T *dict_copy(dict_T *orig, int deep, int top, int copyID);
 int dict_wrong_func_name(dict_T *d, typval_T *tv, char_u *name);