testutil: Make SETUP_TEST_FIXTURE return 0 on fixture == NULL

Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/11808)

test/cmp_hdr_test.c

diff --git a/test/cmp_hdr_test.c b/test/cmp_hdr_test.c
index cd30c49..24299ab 100644
--- a/test/cmp_hdr_test.c
+++ b/test/cmp_hdr_test.c
@@ -402,9 +402,10 @@
 
 static int test_HDR_init_with_ref(void)
 {
-    SETUP_TEST_FIXTURE(CMP_HDR_TEST_FIXTURE, set_up);
     unsigned char ref[CMP_TEST_REFVALUE_LENGTH];
 
+    SETUP_TEST_FIXTURE(CMP_HDR_TEST_FIXTURE, set_up);
+
     fixture->expected = 1;
     if (!TEST_int_eq(1, RAND_bytes(ref, sizeof(ref)))
             || !TEST_true(OSSL_CMP_CTX_set1_referenceValue(fixture->cmp_ctx,
@@ -418,9 +419,9 @@
 
 static int test_HDR_init_with_subject(void)
 {
-    SETUP_TEST_FIXTURE(CMP_HDR_TEST_FIXTURE, set_up);
     X509_NAME *subject = NULL;
 
+    SETUP_TEST_FIXTURE(CMP_HDR_TEST_FIXTURE, set_up);
     fixture->expected = 1;
     if (!TEST_ptr(subject = X509_NAME_new())
             || !TEST_true(X509_NAME_ADD(subject, "CN", "Common Name"))