mavlink_mission: fix dm_read check
mavlink_mission: fix dm_read check

... and init to 0 is not needed, since already done in constructor

mje-nz/PX4-Firmware,dagar/Firmware,krbeverx/Firmware,mcgill-robotics/Firmware,jlecoeur/Firmware,mcgill-robotics/Firmware,mje-nz/PX4-Firmware,Aerotenna/Firmware,acfloria/Firmware,Aerotenna/Firmware,dagar/Firmware,krbeverx/Firmware,PX4/Firmware,jlecoeur/Firmware,Aerotenna/Firmware,Aerotenna/Firmware,jlecoeur/Firmware,krbeverx/Firmware,mje-nz/PX4-Firmware,krbeverx/Firmware,mje-nz/PX4-Firmware,jlecoeur/Firmware,acfloria/Firmware,mcgill-robotics/Firmware,PX4/Firmware,jlecoeur/Firmware,Aerotenna/Firmware,mcgill-robotics/Firmware,dagar/Firmware,PX4/Firmware,krbeverx/Firmware,mcgill-robotics/Firmware,mcgill-robotics/Firmware,dagar/Firmware,mje-nz/PX4-Firmware,acfloria/Firmware,krbeverx/Firmware,PX4/Firmware,dagar/Firmware,PX4/Firmware,jlecoeur/Firmware,krbeverx/Firmware,PX4/Firmware,mcgill-robotics/Firmware,mje-nz/PX4-Firmware,acfloria/Firmware,acfloria/Firmware,Aerotenna/Firmware,dagar/Firmware,jlecoeur/Firmware,acfloria/Firmware,jlecoeur/Firmware,PX4/Firmware,dagar/Firmware,mje-nz/PX4-Firmware,Aerotenna/Firmware,acfloria/Firmware