Fix the notification pipe for squashfuse in case of failure

The following scenario now correctly sends a failure notification to the
pipe:
```
$ mkfifo /tmp/notify_pipe
$ cat /tmp/notify_pipe
```
then run squashfuse:
```
$ ./squashfuse -o notify_pipe=/tmp/notify_pipe /dev/null /mnt
```
This fix only applies to squashfuse, squashfuse_ll was behaving as
expected.

Fixes #112

hl.c

@@ -343,19 +343,21 @@ int main(int argc, char *argv[]) {
 	}
 
 	hl = sqfs_hl_open(opts.image, opts.offset, opts.subdir);
-	if (!hl)
-		return -1;
+	if (!hl) {
+		ret = -1;
+		goto out;
+	}
 
 	hl->fs.notify_pipe = opts.notify_pipe;
 
 	fuse_opt_add_arg(&args, "-s"); /* single threaded */
 	ret = fuse_main(args.argc, args.argv, &sqfs_hl_ops, hl);
+out:
 	if (ret) {
-		if (hl->fs.notify_pipe) {
-			notify_mount_ready(hl->fs.notify_pipe, NOTIFY_FAILURE);
+		if (opts.notify_pipe) {
+			notify_mount_ready(opts.notify_pipe, NOTIFY_FAILURE);
 		}
 	}
-out:
 	fuse_opt_free_args(&args);
 	return ret;
 }