Consistency. placeholder is inited to null in Thread class.

src/General/Build.coffee

@@ -165,7 +165,7 @@ Build =
   thread: (thread, data) ->
     if (root = thread.nodes.root)
       $.rmAll root
-      delete thread.nodes.placeholder
+      thread.nodes.placeholder = null
     else
       thread.nodes.root = root = $.el 'div',
         className: 'thread'

src/General/Index.coffee

@@ -715,7 +715,7 @@ Index =
     for thread in threads
       if thread.nodes.placeholder
         $.replace thread.nodes.placeholder, thread.OP.nodes.root
-        delete thread.nodes.placeholder
+        thread.nodes.placeholder = null
       if (file = thread.OP.file) and (thumb = thread.OP.file.thumb) and thumb.dataset.src
         thumb.src = thumb.dataset.src
         # XXX https://bugzilla.mozilla.org/show_bug.cgi?id=1021289