selftests/bpf: Check malloc result with ASSERT_NEQ in test_loader

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 test_loader.c:6:
    test_loader.c: In function ‘verify_stderr’:
    /bpf-next/tools/testing/selftests/bpf/test_progs.h:393:22: error: ‘buf’ may be used uninitialized [-Werror=maybe-uninitialized]
      393 |         int ___err = libbpf_get_error(___res);                          \
          |                      ^~~~~~~~~~~~~~~~~~~~~~~~
    test_loader.c:810:14: note: in expansion of macro ‘ASSERT_OK_PTR’
      810 |         if (!ASSERT_OK_PTR(buf, "malloc"))
          |              ^~~~~~~~~~~~~
    In file included from /bpf-next/tools/testing/selftests/bpf/tools/include/bpf/bpf.h:32,
                     from /bpf-next/tools/testing/selftests/bpf/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: 554e4eb9e4 ("selftests/bpf: Reuse stderr parsing for libarena ASAN tests")
Signed-off-by: Viktor Malik <vmalik@redhat.com>
Link: https://lore.kernel.org/bpf/e25d50805fbcb3632f24b488568ab5ba49b82094.1784112948.git.vmalik@redhat.com
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

tools/testing/selftests/bpf/test_loader.c

@@ -807,7 +807,7 @@ static void verify_stderr(int prog_fd, struct expected_msgs *msgs)
 		return;
 
 	buf = malloc(TEST_LOADER_LOG_BUF_SZ);
-	if (!ASSERT_OK_PTR(buf, "malloc"))
+	if (!ASSERT_NEQ(buf, NULL, "malloc"))
 		return;
 
 	ret = bpf_prog_stream_read(prog_fd, 2, buf, TEST_LOADER_LOG_BUF_SZ - 1,