fix sf0x: check if device port is given
fix sf0x: check if device port is given

otherwise we risk dereferencing a null pointer

PX4/Firmware,PX4/Firmware,acfloria/Firmware,krbeverx/Firmware,acfloria/Firmware,krbeverx/Firmware,krbeverx/Firmware,PX4/Firmware,acfloria/Firmware,acfloria/Firmware,krbeverx/Firmware,PX4/Firmware,acfloria/Firmware,PX4/Firmware,acfloria/Firmware,PX4/Firmware,PX4/Firmware,acfloria/Firmware,krbeverx/Firmware,krbeverx/Firmware,krbeverx/Firmware