selftests/bpf: Check malloc result with ASSERT_NEQ in test_sha256

Replace ASSERT_OK_PTR by ASSERT_NEQ(res, NULL, ...) when checking the
result of malloc. It is more accurate since malloc returns NULL, not an
error code, on failure and it also prevents the following false GCC
warning when compiling BPF selftests with -O2:

    In file included from /bpf-next/tools/testing/selftests/bpf/prog_tests/sha256.c:4:
    /bpf-next/tools/testing/selftests/bpf/prog_tests/sha256.c: In function ‘test_sha256’:
    ./test_progs.h:393:22: error: ‘data’ may be used uninitialized [-Werror=maybe-uninitialized]
      393 |         int ___err = libbpf_get_error(___res);                          \
          |                      ^~~~~~~~~~~~~~~~~~~~~~~~
    /bpf-next/tools/testing/selftests/bpf/prog_tests/sha256.c:28:14: note: in expansion of macro ‘ASSERT_OK_PTR’
       28 |         if (!ASSERT_OK_PTR(data, "malloc"))
          |              ^~~~~~~~~~~~~
    In file included from /bpf-next/tools/testing/selftests/bpf/tools/include/bpf/bpf.h:32,
                     from ./test_progs.h:37:
    /bpf-next/tools/testing/selftests/bpf/tools/include/bpf/libbpf_legacy.h:113:17: note: by argument 1 of type ‘const void *’ to ‘libbpf_get_error’ declared here
      113 | LIBBPF_API long libbpf_get_error(const void *ptr);
          |                 ^~~~~~~~~~~~~~~~

Fixes: f09f57c746 ("selftests/bpf: Add test for libbpf_sha256()")
Signed-off-by: Viktor Malik <vmalik@redhat.com>
Link: https://lore.kernel.org/bpf/f9dec09cca0c2aa5eeb4fdcd400a13aa19e2c073.1784112948.git.vmalik@redhat.com
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

tools/testing/selftests/bpf/prog_tests/sha256.c

@@ -25,10 +25,10 @@ void test_sha256(void)
 	size_t i;
 
 	data = malloc(MAX_LEN);
-	if (!ASSERT_OK_PTR(data, "malloc"))
+	if (!ASSERT_NEQ(data, NULL, "malloc"))
 		goto out;
 	digests = malloc((MAX_LEN + 1) * SHA256_DIGEST_LENGTH);
-	if (!ASSERT_OK_PTR(digests, "malloc"))
+	if (!ASSERT_NEQ(digests, NULL, "malloc"))
 		goto out;
 
 	/* Generate MAX_LEN bytes of "random" data deterministically. */