org-footnote: Fix LaTeX export error when a src block produces fake footnotes

* lisp/org-footnote.el (org-footnote-delete-definitions): Fix LaTeX
  export error when a src block produces fake footnotes.

lisp/org-footnote.el

@@ -864,8 +864,9 @@ Return the number of footnotes removed."
 	  (ndef 0))
       (while (re-search-forward def-re nil t)
 	(let ((full-def (org-footnote-at-definition-p)))
-	  (delete-region (nth 1 full-def) (nth 2 full-def)))
-	(incf ndef))
+	  (when full-def
+	    (delete-region (nth 1 full-def) (nth 2 full-def))
+	    (incf ndef))))
       ndef)))
 
 (defun org-footnote-delete (&optional label)