nua_server.c: don't assert on sr w/o msg status 500
nua_server.c: don't assert on sr w/o msg status 500

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