Marking from terminated fibers are not needed; ref #4143

The old condition marks the top-level callinfo even after the
fiber is terminated.

src/gc.c

@@ -801,10 +801,10 @@ obj_free(mrb_state *mrb, struct RBasic *obj, int end)
       struct mrb_context *c = ((struct RFiber*)obj)->cxt;
 
       if (c && c != mrb->root_c) {
-        mrb_callinfo *ci = c->ci;
-        mrb_callinfo *ce = c->cibase;
+        if (!end && c->status != MRB_FIBER_TERMINATED) {
+          mrb_callinfo *ci = c->ci;
+          mrb_callinfo *ce = c->cibase;
 
-        if (!end) {
           while (ce <= ci) {
             struct REnv *e = ci->env;
             if (e && !is_dead(&mrb->gc, e) &&