Fix CRYPTO_strdup

The function CRYPTO_strdup (aka OPENSSL_strdup) fails to check the return
value from CRYPTO_malloc to see if it is NULL before attempting to use it.
This patch adds a NULL check.

RT3786

Signed-off-by: Matt Caswell <matt@openssl.org>
(cherry picked from commit 37b0cf936744d9edb99b5dd82cae78a7eac6ad60)

Reviewed-by: Rich Salz <rsalz@openssl.org>
(cherry picked from commit 20d21389c8b6f5b754573ffb6a4dc4f3986f2ca4)