Fix: release reference to ltt_session on error instead of free()

Since ltt_session objects within the session daemon are now
reference counted, it is more appropriate to release a reference
on error rather than calling free() directly in session_create().

The session_release() function also performs additional that can
be needed in some of the error paths of session_create().

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>