nua_client_request: keep own reference to nua handle
nua_client_request: keep own reference to nua handle

darcs-hash:20080521144006-db55f-1d3ceb419d79272509439238698390d1b68adb67.gz

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