ec/curve448/f_generic.c: fix VC-WIN32 debug build failure.

Debugging asserts had implicit casts that triggered the warnings.
However, instead of making the casts explicit it's more appropriate
to perform checks that ensure that implicit casts were safe.

ec/curve448/scalar.c: size_t-fy scalar_decode_short.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5494)