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)

test/ct_test.c

@@ -449,6 +449,9 @@ static int test_encode_tls_sct(void)
     SETUP_CT_TEST_FIXTURE();
 
     fixture->sct_list = sk_SCT_new_null();
+    if (fixture->sct_list == NULL)
+	    return 0;
+
     if (!TEST_ptr(sct = SCT_new_from_base64(SCT_VERSION_V1, log_id,
                                             CT_LOG_ENTRY_TYPE_X509, timestamp,
                                             extensions, signature)))