vim-patch:8.1.1007: using closure may consume a lot of memory

Problem:    Using closure may consume a lot of memory.
Solution:   unreference items that are no longer needed. Add a test. (Ozaki
            Kiichi, closes vim/vim#3961)
209b8e3e3b

src/nvim/eval.c

@@ -442,7 +442,8 @@ void eval_clear(void)
   // unreferenced lists and dicts
   (void)garbage_collect(false);
 
-  // functions
+  // functions need to be freed before gargabe collecting, otherwise local
+  // variables might be freed twice.
   free_all_functions();
 }