bug#23365: 25.0.93; org-mode fails on export with include

[Phillip Lord]

Emacs fails in org-mode when exporting; testing from current head.

The error is reproducible with emacs -q. Create the following two files.

==== main.org

* Section A

#+include: "a.org"

====

==== a.org

** Section of A

Here is some text

====

Now run C-cC-e h h

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  file-exists-p(nil)
  org-check-agenda-file(nil)
  org-agenda-prepare-buffers((nil))
  org-map-entries(#[nil "\300\301 !\207" [org-reduced-level org-current-level] 
2])
  org-export--prepare-file-contents("/home/phillord/scratch/temp-org/a.org" nil 
0 2)
  org-export-expand-include-keyword()
  org-export-as(html nil nil nil (:output-file "./main.html"))
  org-export-to-file(html "./main.html" nil nil nil nil nil)
  org-html-export-to-html(nil nil nil nil)
  org-export-dispatch(nil)
  funcall-interactively(org-export-dispatch nil)
  call-interactively(org-export-dispatch nil nil)
  command-execute(org-export-dispatch)

The error comes from org-map-entries and is not specific to HTML output;
it happens with LaTeX also.