sip_session: minor refactor

darcs-hash:20090415090626-b2873-c6ede507f6ec8f0352f18bb60a44f595ca9ce48b
sip_session: minor internal change

darcs-hash:20090415094509-b2873-6825b49db47354d7b7c60f95d25d7a15bbb852b4