BUG: footnote conflicts with code export to pdf

[zwz <zhangweize@gmail.com>]

Steps to reproduce it:

This org file can be exported to pdf correctly.
#+begin_src org
* test
  #+BEGIN_SRC c
  void main(){
    int a;
  }
  #+END_SRC
#+end

Then you modify it:
#+begin_src org
* test
  #+BEGIN_SRC c
  void main(){
    int a[5];
  }
  #+END_SRC
#+end

It says "org-export-latex-preprocess: Wrong type argument: stringp, nil"
when you try to export the file.