Every commissioner who approves an official bond, which he knows to be, or which by reasonable diligence he could have discovered to have been, insufficient in the penal sum, or in the security thereof, shall be liable as if he were a surety thereto, and may be sued accordingly by any person having a cause of action on said bond. (1869-70, c. 169, s. 6; Code, s. 1879; Rev., s. 313; C.S., s. 335.)