map-extents and narrowed buffer

Hrvoje Niksic writes:
 > I've just noticed that (map-extents FUNCTION nil nil nil) maps over
 > the extents in the whole buffer, not only the narrowed portion.
 > 
 > I wouldn't argue that it's a bug, but it certainly surprised
 > me.

Me too.  But I think it works that way to match the behavior of
make-extent and set-extend-endpoints which both allow setting the
start/end position of an extent outside the clipping region.