MAVLink: Fix FTP file path handling
MAVLink: Fix FTP file path handling

This was a real issue for long paths and not a flight safety issue for regular users, but could have been an issue for developers trying to use FTP on very deep nested file systems

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