Crash when trying to move non-existent source and --auto-rotation enabled

[dvzrv]

A call to Controller::orient_source_toward_reference with a non-existent id makes SSR crash.
This will happen, when calling Controller::set_source_position (also with non-existent id) and --auto-rotation being enabled (by default).

A prior call to ssr::Scene::get_source_orientation() in Controller::orient_source_toward_reference with the same (non-existent) id and a subsequent check for null_ptr could probably prevent this.

[dvzrv]

#79 fixes this, however, I'm not sure it's the best way. Maybe a first general check to whether the Source is available at all would be better, so subsequent calls to functions can be stopped at that point already?