evp_test: Add the missing check after calling OPENSSL_malloc

evp_test: Add the missing check after calling OPENSSL_malloc

The OPENSSL_zalloc() could return NULL pointer if fails.
Add the check for it does make sense, like how digest_test_init() deals
with.

CLA: trivial

Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17571)

(cherry picked from commit 2208ba56ebefe4cf7d924e2ac7044ccd3307250b)