text/html clickable links fail in p0.63+w3-4.0pre31

[Karl Kleinpaste <karl@justresearch.com>]

A forwarded text/html article in a local newsgroup contains numerous
<a href>'s.  Merely passing the mouse over these links induces this
stack trace:

Signaling: (args-out-of-range #<buffer "*Summary jprc.support.linux*"> #<marker at 1068 in *Article* 0x8947334> #<marker at 1080 in *Article* 0x88e6ec4>)
  buffer-substring-no-properties(#<marker at 1068 in *Article* 0x8947334> #<marker at 1080 in *Article* 0x88e6ec4>)
  w3-widget-echo((link :button-extent #<extent (1068, 1080) H keymap help-echo balloon-help button-or-field button 0x8fc3814 in buffer *Article*> :to #<marker at 1080 in *Article* 0x88e6ec4> :class (":link") :href "http://www.excite.com/" :emacspeak-help w3-widget-echo :help-echo w3-widget-echo :from #<marker at 1068 in *Article* 0x8947334> :visited-face #<face w3-style-face-00021 "An Emacs-W3 face... don't edit by hand."> :active-face #<face w3-style-face-00020 "An Emacs-W3 face... don't edit by hand."> :button-face (nil) :action w3-follow-hyperlink :tag "" :value "" :args nil :end 1083 :start 1071 :delete widget-leave-text))
  widget-echo-this-extent(#<extent (1068, 1080) H keymap help-echo balloon-help button-or-field button 0x8fc3814 in buffer *Article*>)
  default-mouse-motion-handler(#<motion-event 847, 533>)

(I'm experimenting with XEmacs 21.2-b4, so if that's part of the
problem, I suppose this should be ignored...)