ox-md: Fix blank lines insertion in sub-lists

* lisp/ox-md.el (org-md-separate-elements): In a list, when a sub-list
  is followed by another element, make sure it is surrounded by blank
  lines.

Reported-by: dbitouze@wanadoo.fr (Denis Bitouzé)
<http://permalink.gmane.org/gmane.emacs.orgmode/101113>

lisp/ox-md.el

@@ -116,9 +116,10 @@ Assume BACKEND is `md'."
        e :post-blank
        (if (and (eq (org-element-type e) 'paragraph)
 		(eq (org-element-type (org-element-property :parent e)) 'item)
-		(eq (org-element-type (org-export-get-next-element e info))
-		    'plain-list)
-		(not (org-export-get-previous-element e info)))
+		(not (org-export-get-previous-element e info))
+		(let ((next (org-export-get-next-element e info)))
+		  (and (eq (org-element-type next) 'plain-list)
+		       (not (org-export-get-next-element next info)))))
 	   0
 	 1))))
   ;; Return updated tree.