msg_multipart_s: fixed mp_len type
msg_multipart_s: fixed mp_len type

The msg_multipart_s should follow msg_pub_s layout.

darcs-hash:20090121163808-db55f-a3b06dbde8e28ab5fc3672bcb3e9cac5d3ab561a.gz

unispeech/sofia-sip,xhook/sofia-sip,xhook/sofia-sip,jart/sofia-sip,unispeech/sofia-sip,xhook/sofia-sip,xhook/sofia-sip,jart/sofia-sip,BelledonneCommunications/sofia-sip,BelledonneCommunications/sofia-sip,jart/sofia-sip,erdincay/sofia-sip,xhook/sofia-sip,erdincay/sofia-sip,unispeech/sofia-sip,BelledonneCommunications/sofia-sip,erdincay/sofia-sip,unispeech/sofia-sip,erdincay/sofia-sip,BelledonneCommunications/sofia-sip