block_given? should return false on top-level; ref #2678

src/kernel.c

@@ -189,6 +189,9 @@ mrb_f_block_given_p_m(mrb_state *mrb, mrb_value self)
     if (ci->proc->env && ci->proc->env->stack) {
       mrb_value *sp = ci->proc->env->stack;
 
+      /* top-level does not have block slot (alway false) */
+      if (sp == mrb->c->stbase)
+        return mrb_false_value();
       while (mrb->c->cibase < ci) {
         if (ci->stackent == sp) {
           break;