“ That issue, now fairly tendered by the petition for habeas corpus below, has never been adjudicated on its merits by the lower courts. But it is no longer within the bosom of the trial court. Nor can it be disposed of on the appeal of his conviction, for the claim rests on materials dehors the trial proceedings. It is a claim which the relator should be allowed to establish, if he can. We cannot say that, in the light of the supporting affidavits, the petition for a writ of habeas corpus was palpably unmeritorious, and should have been dismissed without more. ”