clear any additional response on smtp connect

When connecting to an SMTP server after `HELO` and auth
complete there can be additional data sent from the client
that sits in the socket queue. Adding a `get_once` after connection
has settled ensure any pending for extension responses are cleared.

lib/msf/core/exploit/remote/smtp_deliver.rb

@@ -244,6 +244,10 @@ module Exploit::Remote::SMTPDeliver
     begin
       nsock.put(cmd)
       res = nsock.get_once
+      # read until `nil` is returned
+      while chunk = nsock.get_once
+        res += chunk
+      end
     rescue
       return nil
     end