Protect against invalid punycode

* lisp/net/puny.el (puny-decode-string): Protect against invalid
punycode.

lisp/net/puny.el

@@ -63,7 +63,10 @@ For instance, \"xn--ff-2sa.org\" => \"fśf.org\"."
   "Decode an IDNA/punycode-encoded string.
 For instance \"xn--bcher-kva\" => \"bücher\"."
   (if (string-match "\\`xn--" string)
-      (puny-decode-string-internal (substring string 4))
+      (condition-case nil
+          (puny-decode-string-internal (substring string 4))
+        ;; If the string is invalid Punycode, just return the string.
+        (args-out-of-range string))
     string))
 
 (defconst puny-initial-n 128)