logger: fix uninitialized member variable
logger: fix uninitialized member variable

This was reported by Coverity Scan.

dagar/Firmware,mje-nz/PX4-Firmware,krbeverx/Firmware,acfloria/Firmware,mje-nz/PX4-Firmware,PX4/Firmware,acfloria/Firmware,dagar/Firmware,PX4/Firmware,dagar/Firmware,krbeverx/Firmware,krbeverx/Firmware,mje-nz/PX4-Firmware,PX4/Firmware,acfloria/Firmware,PX4/Firmware,mje-nz/PX4-Firmware,krbeverx/Firmware,krbeverx/Firmware,mje-nz/PX4-Firmware,krbeverx/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,dagar/Firmware,krbeverx/Firmware,acfloria/Firmware,acfloria/Firmware,dagar/Firmware,acfloria/Firmware,acfloria/Firmware,mje-nz/PX4-Firmware,PX4/Firmware,PX4/Firmware,dagar/Firmware,PX4/Firmware