mac: always pass a non-NULL output size pointer to providers.
mac: always pass a non-NULL output size pointer to providers.

The backend code varies for the different MACs and sometimes sets the output
length, sometimes checks the return pointer and sometimes neither.

Reviewed-by: Richard Levitte <5fb523282dd7956571c80524edc2dccfa0bd8234@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/12458)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl