Need to mark shared env objects as `MRB_TT_FREE`; fix #4143

The following code mistakenly exits from the function without marking
the env object as `MRB_TT_FREE`.

``` ruby
      if (MRB_ENV_STACK_SHARED_P(e)) {
        /* cannot be freed */
        return; // <- should be `break`
      }
```

src/gc.c

@@ -788,7 +788,8 @@ obj_free(mrb_state *mrb, struct RBasic *obj, int end)
 
       if (MRB_ENV_STACK_SHARED_P(e)) {
         /* cannot be freed */
-        return;
+        e->stack = NULL;
+        break;
       }
       mrb_free(mrb, e->stack);
       e->stack = NULL;