mac poly1305: add missing NULL check in new function.
mac poly1305: add missing NULL check in new function.

Bug reported by Kihong Heo.

Reviewed-by: Richard Levitte <5fb523282dd7956571c80524edc2dccfa0bd8234@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10613)

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