fix(realtime): aboard finishConnection if user is missing

This can happen if the user disconnects during connection creation.

See #6101

Signed-off-by: Philip Molares <philip.molares@udo.edu>

lib/realtime.js

@@ -384,6 +384,9 @@ function finishConnection (socket, noteId, socketId) {
   }
   const note = notes[noteId]
   const user = users[socketId]
+  if (!user) {
+    return failConnection(500, 'user not found', socket)
+  }
   // update user color to author color
   if (note.authors[user.userid]) {
     user.color = users[socket.id].color = note.authors[user.userid].color