bug#3768: org: C-c C-x C-l: Wrong type argument: stringp, nil

[David Reitter]

- Start Emacs (23 series, recent build),
- M-x org-mode  (for the scratch buffer),
- insert  $\frac{3}{a*b-3}$
- press C-c C-x C-l
- Unhelpful error message "Wrong type argument: stringp, nil"

This goes away when the buffer is given a buffer-file-name.

The error message should be either helpful or not appear at all - org- 
mode should use a temporary directory instead of the documents  
directory to store temporary files in order to render LaTeX content.