Make sure OPENSSL_cleanse checks for NULL

In master we have the function OPENSSL_clear_free(x,y), which immediately
returns if x == NULL. In <=1.0.2 this function does not exist so we have to
do:
OPENSSL_cleanse(x, y);
OPENSSL_free(x);

However, previously, OPENSSL_cleanse did not check that if x == NULL, so
the real equivalent check would have to be:
if (x != NULL)
    OPENSSL_cleanse(x, y);
OPENSSL_free(x);

It would be easy to get this wrong during cherry-picking to other branches
and therefore, for safety, it is best to just ensure OPENSSL_cleanse also
checks for NULL.

Reviewed-by: Rich Salz <rsalz@openssl.org>