outline.el fix for bug#7619.

outline.el fix for bug#7619.

It turns out the use of nil in the :type of outline regexp was for
very hysterical raisons.  Prior to CVS rev 1.49, 2001-09-12, the
default value was nil, then it was initialized afterwards.
Everywhere in the code assumes it cannot be nil, so no need to allow
that any more.

* lisp/outline.el (outline-regexp): No longer allow nil.
(outline-heading-end-regexp): Add safety predicate.