patch 9.0.0487: using freed memory with combination of closures

Problem:    Using freed memory with combination of closures.
Solution:   Do not use a partial after it has been freed through the
            funcstack.
diff --git a/src/proto/vim9execute.pro b/src/proto/vim9execute.pro
index a2b56c8..708f69a 100644
--- a/src/proto/vim9execute.pro
+++ b/src/proto/vim9execute.pro
@@ -1,7 +1,7 @@
 /* vim9execute.c */
 void to_string_error(vartype_T vartype);
 void update_has_breakpoint(ufunc_T *ufunc);
-void funcstack_check_refcount(funcstack_T *funcstack);
+int funcstack_check_refcount(funcstack_T *funcstack);
 int set_ref_in_funcstacks(int copyID);
 int in_def_function(void);
 ectx_T *clear_currrent_ectx(void);