Don't pass vanilla plugin channels to plugins. Fixes BUKKIT-2638

Vanilla has its own handlers for plugin channel messages for things like
texture packs, books, and anvils. When vanilla handles one of these messages
we should not also pass it to plugins because they will be duplicating work
and potentially running in to situations our plugin system isn't setup to
handle. This is how 1.3.2 worked but was lost in the 1.4.2 update.

src/main/java/net/minecraft/server/NetServerHandler.java

@@ -1509,11 +1509,8 @@ public class NetServerHandler extends NetHandler {
                         containeranvil.a("");
                     }
                 }
-            }
-        }
-
                 // CraftBukkit start
-        if (packet250custompayload.tag.equals("REGISTER")) {
+                else if (packet250custompayload.tag.equals("REGISTER")) {
                     try {
                         String channels = new String(packet250custompayload.data, "UTF8");
                         for (String channel : channels.split("\0")) {
@@ -1537,3 +1534,5 @@ public class NetServerHandler extends NetHandler {
                 // CraftBukkit end
             }
         }
+    }
+}