Fix #79413: session_create_id() fails for active sessions

Fix #79413: session_create_id() fails for active sessions

The comment on `PS_VALIDATE_SID_FUNC(files)` is very clear that the
function is supposed to return `SUCCESS` if the session already exists.
So to detect a collision, we have to check for `SUCCESS`, not
`FAILURE`.

We also fix the wrong condition in session_regenerate_id() as well.