Clean up and fix pkey_cmac_keygen()

Clean up and fix pkey_cmac_keygen()

A void pointer can be passed without any cast or assigning it to an
intermediate variable. That's one of hte puzzling things in old OpenSSL
code: there are plenty of unnecessary casts and assignments of void
pointers.

Make use of this fact and rework the function to be single exit, error
check consistently, including the EVP_PKEY_assign() call that can't
really fail and free the cmkey on exit.

Why coverity didn't flag this one is another mystery.

ok jsing