MINOR: session: use conn->src instead of conn->addr.from

MINOR: session: use conn->src instead of conn->addr.from

In session_accept_fd() we'll soon have to dynamically allocate the
address, or better, steal it from the caller and define a strict calling
convention regarding who's responsible for the freeing. In the simpler
session_prepare_log_prefix(), just add an attempt to retrieve the address
if not yet set and do not dereference it on failure.