Use LPORT if opts[:lport] is undefined

`nil.to_i` returns 0 which will short circuit the || resulting in port 0
being used. nil should be checked for prior to casting to int.

lib/msf/core/payload/transport_config.rb

@@ -56,7 +56,7 @@ module Msf::Payload::TransportConfig
     {
       :scheme       => 'http',
       :lhost        => opts[:lhost] || datastore['LHOST'],
-      :lport        => opts[:lport].to_i || datastore['LPORT'].to_i,
+      :lport        => (opts[:lport] || datastore['LPORT']).to_i,
       :uri          => uri,
       :comm_timeout => datastore['SessionCommunicationTimeout'].to_i,
       :retry_total  => datastore['SessionRetryTotal'].to_i,