Code review comment for lp:~widelands-dev/widelands/ferry

I get a compiler error on 8780:

../src/logic/map_objects/tribes/ferry.cc: In member function ‘void Widelands::Ferry::start_task_row(Widelands::Game&, Widelands::Waterway*)’:
../src/logic/map_objects/tribes/ferry.cc:106:17: error: invalid conversion from ‘const value_type*’ {aka ‘const Widelands::Coords*’} to ‘Widelands::Coords*’ [-fpermissive]
  destination_ = &(CoordPath(game.map(), ww->get_path()).get_coords()[ww->get_idle_index()]);