Avoid tracing shared TT_ENV object.
@@ -639,6 +639,7 @@ 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]);
