evp_test: use correct size in memory clear
evp_test: use correct size in memory clear

Fixes #15917

Reviewed-by: Richard Levitte <5fb523282dd7956571c80524edc2dccfa0bd8234@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15920)

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