chat: in addMessage(), be tolerant when userId is missing

For whatever reason (a bug, a database corruption, ...) the userId field in
"msg" can sometimes be missing.

In this case, let's be defensive, use "unknown" as userId and issue a warning
in the console, instead of crashing the client.

Fixes #3731 (really a patch, the underlying issue is still present)

src/static/js/chat.js

@@ -132,6 +132,16 @@ var chat = (function()
       var timeStr = hours + ":" + minutes;
 
       //create the authorclass
+      if (!msg.userId) {
+        /*
+         * If, for a bug or a database corruption, the message coming from the
+         * server does not contain the userId field (see for example #3731),
+         * let's be defensive and replace it with "unknown".
+         */
+        msg.userId = "unknown";
+        console.warn('The "userId" field of a chat message coming from the server was not present. Replacing with "unknown". This may be a bug or a database corruption.');
+      }
+
       var authorClass = "author-" + msg.userId.replace(/[^a-y0-9]/g, function(c)
       {
         if (c == ".") return "-";