[1.5.x] Added compatibility with legacy message length

Added compatibility code to not break when decoding messages
encoded on pre-1.5 versions.
Refs #19387. Thanks Florian Apolloner for noticing the issue.

django/contrib/messages/storage/cookie.py

@@ -33,6 +33,9 @@ class MessageDecoder(json.JSONDecoder):
     def process_messages(self, obj):
         if isinstance(obj, list) and obj:
             if obj[0] == MessageEncoder.message_key:
+                if len(obj) == 3:
+                    # Compatibility with previously-encoded messages
+                    return Message(*obj[1:])
                 if obj[1]:
                     obj[3] = mark_safe(obj[3])
                 return Message(*obj[2:])