mml: Signal error on invalid <#secure> tag

  
* mml.el (mml-parse-1): Error out if unknown mode is specified in
<#secure> tag (bug#18513).

lisp/gnus/ChangeLog

+2014-09-29  Daiki Ueno  <ueno@gnu.org>
+
+	* mml.el (mml-parse-1): Error out if unknown mode is specified in
+	<#secure> tag (bug#18513).
+
 2014-06-26  Glenn Morris  <rgm@gnu.org>
 	* mm-util.el (help-function-arglist): Remove outdated declaration.

lisp/gnus/mml.el

@@ -260,7 +260,9 @@ part.  This is for the internal use, you should never modify the value.")
 		((string= mode "encrypt")
 		 (setq tags (list "encrypt" method)))
 		((string= mode "signencrypt")
-		 (setq tags (list "sign" method "encrypt" method))))
+		 (setq tags (list "sign" method "encrypt" method)))
+		(t
+		 (error "Unknown secure mode %s" mode)))
 	  (eval `(mml-insert-tag ,secure-mode
 				 ,@tags
 				 ,(if keyfile "keyfile")