Assertions can be disabled, but at the point where cryptographic
anything is involved, a single branch has an infinitesimally small
performance impact.
The correct thing to do is to punch the caller in the face if they do
something that is blatantly incorrect, especially in a security critical
setting.
The `update` and `final` routines were written with the assumption that
update will only be called once, and that the underlying data does not
change between the calls.
