“ Normally, allowance of an appeal should not be denied until an indigent has had adequate representation by counsel. Johnson v. United States, 352 U.S. 565, 77 S.Ct. 550, 1 L.Ed.2d 593. In this case, it appears that the two attorneys appointed by the Court of Appeals, performed essentially the role of amici curiae. But representation in the role of an advocate is required. If counsel is convinced, after conscientious investigation, that the appeal is frivolous, of course, he may ask to withdraw on that account. ”