Any person who escapes from the custody of the commissioner of corrections, regardless of where such person is confined or where such escape occurs, is guilty of a felony, and, upon conviction thereof, shall be imprisoned in the penitentiary not more than five years. A term of imprisonment imposed pursuant to the provisions of this section shall be imposed as a consecutive sentence and shall not be served concurrently with any imprisonment, confinement or detention imposed under any prior sentence being served or otherwise being discharged at the time such person commits an offense under the provisions of this section. A person charged with an offense under the provisions of this section shall not be released from the custody of the commissioner of corrections while the prosecution of the alleged offense is pending: Provided, That time served by such person after any other prior sentence has been served or otherwise discharged shall be applied to any sentence which may ultimately be imposed for an offense under this section. Venue for the prosecution of a violation of this section shall be in the county in which the escape occurs.