When the cause alleged for not pronouncing sentence is that the defendant has been pardoned for the offense for which the defendant is about to be sentenced, the court, if necessary, shall postpone the pronouncement of sentence for the purpose of hearing evidence on the allegation. If the court decides that the allegation is true, it shall discharge the defendant from custody unless the defendant is in custody on some other charge. If, however, it decides that the allegation is not true, it shall proceed to pronounce sentence.