Clear unused stack region that may refer freed objects; fix #3596

src/gc.c

@@ -544,6 +544,7 @@ mark_context_stack(mrb_state *mrb, struct mrb_context *c)
 {
   size_t i;
   size_t e;
+  mrb_value nil;
 
   if (c->stack == NULL) return;
   e = c->stack - c->stbase;
@@ -553,13 +554,13 @@ mark_context_stack(mrb_state *mrb, struct mrb_context *c)
     mrb_value v = c->stbase[i];
 
     if (!mrb_immediate_p(v)) {
-      if (mrb_basic_ptr(v)->tt == MRB_TT_FREE) {
-        c->stbase[i] = mrb_nil_value();
-      }
-      else {
       mrb_gc_mark(mrb, mrb_basic_ptr(v));
     }
   }
+  e = c->stend - c->stbase;
+  nil = mrb_nil_value();
+  for (; i<e; i++) {
+    c->stbase[i] = nil;
   }
 }