No part of the stock of any bank except regional reserve banks shall be owned by a state bank. In cases where such stock is taken as collateral and the purchase thereof shall be necessary to prevent loss upon a debt previously contracted in good faith, then in such cases such stock shall be sold by the bank within twelve months from the time it was acquired, unless the consent of the state comptroller is obtained in writing extending such period. A violation of this section by any bank shall subject it to liquidation and forfeiture of charter.