selftests/bpf: Fix check in global_data_init.

The returned value of bpf_object__open_file() should be checked with
libbpf_get_error() rather than NULL. This fix prevents test_progs from
crash when test_global_data.o is not present.
Signed-off-by: Hao Luo <haoluo@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200903200528.747884-1-haoluo@google.com

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

@@ -12,7 +12,8 @@ void test_global_data_init(void)
 	size_t sz;
 
 	obj = bpf_object__open_file(file, NULL);
-	if (CHECK_FAIL(!obj))
+	err = libbpf_get_error(obj);
+	if (CHECK_FAIL(err))
 		return;
 
 	map = bpf_object__find_map_by_name(obj, "test_glo.rodata");