[O] Regression: org-export-as-html failure

[Steve Purcell]

Using the org-mode included in Emacs HEAD as of yesterday, the following
content causes an error when exporting as html:

https://gist.github.com/purcell/5055957

Backtrace:

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  string-match("\\([^\\\\]\\)\\([_^]\\)" nil)
  org-export-protect-sub-super(nil)
  org-export-normalize-links()
  org-export-preprocess-string(...)
  org-export-as-html(nil)
  call-interactively(org-export-as-html)

Export as iCal works fine. The list of bare http links seems to be the
problem, but until recently org-mode happily exported this content to
html.

-Steve