Fix small bug where plugins that don't override on_msg/2 would report errors

Since route_msg expects msg.prefix to be present, every plugin that
calls route_msg when no prefix is specified in the message (e.g. with
PING messages) would cause a crash of the process.

Now, it just calls a no-op function, since all behavior covered by
route_message requires information in the prefix at some point.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

lib/plugin/base.ex

@@ -42,6 +42,8 @@ defmodule Omnibot.Plugin.Base do
         route_msg(irc, msg)
       end
 
+      defp route_msg(irc, %Irc.Msg {prefix: nil}), do: nil
+
       defp route_msg(irc, msg) do
         nick = msg.prefix.nick
         case String.upcase(msg.command) do