tools: bpf: handle NULL return in bpf_prog_load_xattr()

bpf_object__open() can return error pointer as well as NULL.
Fix error handling in bpf_prog_load_xattr() (and indirectly
bpf_prog_load()).

Fixes: 6f6d33f3b3d0 ("bpf: selftests add sockmap tests")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/lib/bpf/libbpf.c

@@ -2035,7 +2035,7 @@ int bpf_prog_load_xattr(const struct bpf_prog_load_attr *attr,
 		return -EINVAL;
 
 	obj = bpf_object__open(attr->file);
-	if (IS_ERR(obj))
+	if (IS_ERR_OR_NULL(obj))
 		return -ENOENT;
 
 	bpf_object__for_each_program(prog, obj) {