test/ct_test.c: Add the missing check after calling sk_SCT_new_null

test/ct_test.c: Add the missing check after calling sk_SCT_new_null

As the potential failure of the allocation, the sk_SCT_new_null() could
return NULL pointer if fails.
And then sk_SCT_push() uses the 'fixture->sct_list' and returns -1 if
fails.
But the return value of the sk_SCT_push() is not checked.
I think it is better to check it just after the allocation.

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)