libbpf: Potential NULL dereference in usdt_manager_attach_usdt()

link could be null but still dereference bpf_link__destroy(&link->link)
and it will lead to a null pointer access.
Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/1649299098-2069-1-git-send-email-baihaowen@meizu.com

tools/lib/bpf/usdt.c

@@ -1071,8 +1071,8 @@ struct bpf_link *usdt_manager_attach_usdt(struct usdt_manager *man, const struct
 	return &link->link;
 
 err_out:
-	bpf_link__destroy(&link->link);
-
+	if (link)
+		bpf_link__destroy(&link->link);
 	free(targets);
 	hashmap__free(specs_hash);
 	if (elf)