fix px4_getopt: correctly handle options that take an argument, but no argument is given
fix px4_getopt: correctly handle options that take an argument, but no argument is given

This lead to a segfault, for example 'logger start -r'

Aerotenna/Firmware,mje-nz/PX4-Firmware,darknight-007/Firmware,krbeverx/Firmware,acfloria/Firmware,krbeverx/Firmware,Aerotenna/Firmware,mcgill-robotics/Firmware,mje-nz/PX4-Firmware,PX4/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,darknight-007/Firmware,PX4/Firmware,mcgill-robotics/Firmware,Aerotenna/Firmware,acfloria/Firmware,dagar/Firmware,dagar/Firmware,mcgill-robotics/Firmware,PX4/Firmware,dagar/Firmware,Aerotenna/Firmware,krbeverx/Firmware,darknight-007/Firmware,mje-nz/PX4-Firmware,mcgill-robotics/Firmware,mcgill-robotics/Firmware,krbeverx/Firmware,acfloria/Firmware,jlecoeur/Firmware,acfloria/Firmware,Aerotenna/Firmware,jlecoeur/Firmware,Aerotenna/Firmware,dagar/Firmware,mje-nz/PX4-Firmware,darknight-007/Firmware,jlecoeur/Firmware,mje-nz/PX4-Firmware,jlecoeur/Firmware,mcgill-robotics/Firmware,mje-nz/PX4-Firmware,jlecoeur/Firmware,krbeverx/Firmware,PX4/Firmware,Aerotenna/Firmware,jlecoeur/Firmware,PX4/Firmware,krbeverx/Firmware,jlecoeur/Firmware,dagar/Firmware,PX4/Firmware,dagar/Firmware,acfloria/Firmware,krbeverx/Firmware,acfloria/Firmware,PX4/Firmware,jlecoeur/Firmware,darknight-007/Firmware,mcgill-robotics/Firmware,acfloria/Firmware