check if the "nasal" subsystem is still there, and only call <unload> then.

check if the "nasal" subsystem is still there, and only call <unload> then.
This isn't the case if the model is destroyed on fgfs exit. To make <unload>
work under these circumstances, one would have to reorder subsystem removal,
but this doesn't seem overly useful, so we'll do it when we really need it.