I get mail from an ietf mailing list that uses multipart/alternative
to wrap two message/external-body subparts.  This type doesn't show up
in `mm-alternative-precedence'.  The message displays ok, with neither
of the alternative parts marked with an asterisk.

However, if I try to toggle the button, either by, say `N b' in the
summary buffer, or by `RET' in the article buffer, I get

	Wrong type argument: integer-or-marker-p, nil

After poking around a bit, I believe the problem is due to the 

	(when preferred
	   ...
	   (setcdr begend (point-max)))

block near the end of gnus-mime-display-alternative.  Because none of
the alternatives was chosen to display, this is not executed.  Because
the setcdr doesn't happen, when gnus-mime-display-alternative is
invoked from the gnus-callback property, narrow-to-region fails in 

	(when ibegend ...)

just inside the save-restriction.  I've attached a traceback at the
end of this message.  Below is an example multipart alternative that
should exhibit the problem.