evp-cmac: do not seg-fault when getting mac-size before init

evp-cmac: do not seg-fault when getting mac-size before init

Add null check to cmac_size().  This avoids a seg-fault encountered
with cmac when EVP_MAC_CTX_get_mac_size() is called before init.

Extend mac testing in evp_test.c to check that the sizes returned by
EVP_MAC_CTX_get_mac_size() before and after init make sense (this also
ensures that we no longer seg-fault).

Fixes #22842

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22858)