Update for change in letrec behavior

Update for change in letrec behavior

* lisp/ox-latex.el (org-latex--delayed-footnotes-definitions):
* lisp/ox.el (org-export--selected-trees):
(org-export--prune-tree):
(org-export--merge-external-footnote-definitions):
(org-export--footnote-reference-map): Provide nil value for binders.

Using

  (letrec ((SYMBOL)) ...)

to bind SYMBOL to nil relies on (setq SYMBOL) setting SYMBOL to nil.  As
of 5d93a89 (Byte compiler: on setq with an odd number of arguments,
generate a `signal', 2015-11-26) in the Emacs repo, setq signals an
error when given an odd number of arguments.

Reported-by: Zack Piper <zack@apertron.net>
<http://permalink.gmane.org/gmane.emacs.orgmode/103539>
<http://permalink.gmane.org/gmane.emacs.devel/196182>