nua_register.c: nr->nr_tport was used even if nr was NULL
nua_register.c: nr->nr_tport was used even if nr was NULL

Problem detected by Klocwork.

darcs-hash:20080516090609-65a35-ee7d52f92fbc8d38046ed33d95ca13f94fe50482.gz

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