Revert 6dd1a570; fix #3612; ref #3550

The fix was wrong. It causes a new problem #3612.

src/gc.c

@@ -646,6 +646,7 @@ gc_mark_children(mrb_state *mrb, mrb_gc *gc, struct RBasic *obj)
       struct REnv *e = (struct REnv*)obj;
       mrb_int i, len;
 
+      if (MRB_ENV_STACK_SHARED_P(e)) break;
       len = MRB_ENV_STACK_LEN(e);
       for (i=0; i<len; i++) {
         mrb_gc_mark_value(mrb, e->stack[i]);