Changeset 7d6a751 for perl

Move cachedmsgid from owl_filter to owl_view.

This fixes a use-after-free bug: owl_function_create_filter sets
inuse=1, removes the current filter (hence freeing it), then calls
owl_function_change_currentview_filter → owl_view_save_curmsgid →
owl_filter_set_cachedmsgid, which writes to the filter that was just
freed.

This means that you can no longer keep two independent positions in
two different filters by repeatedly switching between filter1 → empty
→ filter2 → empty → filter1.  But I doubt anyone even knew that was
possible before.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Reviewed-by: Nelson Elhage <nelhage@mit.edu>