sip_session: minor comment change

darcs-hash:20090330145626-b2873-cffeedf9d6b884b6aeb81a69cc2182cb0ea5a3ad
sip_session: protect against ValueError

darcs-hash:20090330145636-b2873-61d551d1c111e4ee8f56e4eb9408ede7e4c99755