Backport commit c5e02f2bc from Emacs

* lisp/org-protocol.el (org-protocol-flatten):
Rewrite as top-level alias, as per Stefan’s suggestion,
to avoid compiler warnings.

Make org-protocol-flatten always an alias
c5e02f2bce28f3b1f2006ce1f208f4a92ca05ed9
Paul Eggert
Mon Dec 17 13:32:52 2018 -0800

lisp/org-protocol.el

@@ -350,20 +350,20 @@ returned list."
 	  ret)
       l)))
 
-(if (fboundp 'flatten-tree)
-    (defalias 'org-protocol-flatten 'flatten-tree)
-  (defun org-protocol-flatten (list)
-    "Transform LIST into a flat list.
+(defalias 'org-protocol-flatten
+  (if (fboundp 'flatten-tree) 'flatten-tree
+    (lambda (list)
+      "Transform LIST into a flat list.
 
 Greedy handlers might receive a list like this from emacsclient:
 \((\"/dir/org-protocol:/greedy:/~/path1\" (23 . 12)) (\"/dir/param\"))
 where \"/dir/\" is the absolute path to emacsclients working directory.
 This function transforms it into a flat list."
-    (if (null list) ()
-      (if (listp list)
-	  (append (org-protocol-flatten (car list))
-                  (org-protocol-flatten (cdr list)))
-        (list list)))))
+      (if list
+	  (if (consp list)
+	      (append (org-protocol-flatten (car list))
+		      (org-protocol-flatten (cdr list)))
+	    (list list))))))
 
 (defun org-protocol-parse-parameters (info &optional new-style default-order)
   "Return a property list of parameters from INFO.