fix(lib/s2tt): additional asserts in s2tt library

This patch adds additional asserts in s2tt library related
mostly to incomplete checks.

Signed-off-by: Javier Almansa Sobrino <javier.almansasobrino@arm.com>
Change-Id: Ib73d394ed96b53db79992b4955d5a8fe39f26bb2