A person of an offence before a court-martial shall be a competent witness for the defence and may give evidence on oath in disproof of the charges made against him or any person charged together with him at the same trial:
    Provided that--
         (a) he shall not be called as a witness except on his own request in writing; or
         (b) his failure to give evidence shall not be made the subject of any comment by any of the parties or the court or give rise to any presumption against himself or any person charged together with him at the same trial.