“ Although petitioner received a hearing on the issue of his competence to stand trial, there appears to have been no hearing or inquiry into the issue of his competence to waive his constitutional right to the assistance of counsel and proceed, as he did, to conduct his own defense. 'The constitutional right of an accused to be represented by counsel invokes, of itself, the protection of a trial court, in which the accused-whose life or liberty is at stake-is without counsel. ”