Fix the parameter type of gf_serialize

Fix the parameter type of gf_serialize

It is better to use array bounds for improved
gcc warning checks.

While "uint8_t*" allows arbitrary pointer arithmetic
using "uint8_t[SER_BYTES]" limits the pointer arithmetic
to the range 0..SER_BYTES.

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16376)