Fix a bug for allowed paths

Fix a bug for allowed paths

Under some conditions on my system
aircraft_paths.begin() was equal to scenery_paths.end()
This resulted in neither of them being added to read_allowed_paths
followed by failure to load Nasal scripts from the aircraft directory.

This might happen, if scenery_paths gets allocated just before aircraft_paths
in memory.
Better loop across the desired lists instead of using fancy tricks with
the iterator.