Clear env before top-level execution; fix #4581

src/vm.c

@@ -2834,6 +2834,7 @@ mrb_top_run(mrb_state *mrb, struct RProc *proc, mrb_value self, unsigned int sta
     return mrb_vm_run(mrb, proc, self, stack_keep);
   }
   if (mrb->c->ci == mrb->c->cibase) {
+    mrb->c->ci->env = NULL;
     return mrb_vm_run(mrb, proc, self, stack_keep);
   }
   ci = cipush(mrb);