Fix exporting LaTeX fragments within inline footnotes

* lisp/ox.el (org-export--prune-tree): Do not change parent by side
  effect. It affects footnote references already in the tree.

Reported-by: Paul Stansell <paulstansell@gmail.com>
<http://lists.gnu.org/archive/html/emacs-orgmode/2017-08/msg00302.html>

lisp/ox.el

@@ -2712,14 +2712,11 @@ from tree."
 	     (org-element-map data '(footnote-definition footnote-reference)
 	       (lambda (f)
 		 (cond
-		  ((eq (org-element-type f) 'footnote-definition) f)
-		  ((eq (org-element-property :type f) 'standard) nil)
-		  (t (let ((label (org-element-property :label f)))
-		       (when label	;Skip anonymous references.
-			 (apply
-			  #'org-element-create
-			  'footnote-definition `(:label ,label :post-blank 1)
-			  (org-element-contents f))))))))))
+		  ((eq 'footnote-definition (org-element-type f)) f)
+		  ((and (eq 'inline (org-element-property :type f))
+			(org-element-property :label f))
+		   f)
+		  (t nil))))))
     ;; If a select tag is active, also ignore the section before the
     ;; first headline, if any.
     (when selected