nspawn: guard acl_free() with a NULL check

Inspired by #25957 there's one other place where we don't guard
acl_free() calls with a NULL check.

Fix that.

src/nspawn/nspawn-patch-uid.c

@@ -181,7 +181,9 @@ static int patch_acls(int fd, const char *name, const struct stat *st, uid_t shi
 
         if (S_ISDIR(st->st_mode)) {
                 acl_free(acl);
-                acl_free(shifted);
+
+                if (shifted)
+                        acl_free(shifted);
 
                 acl = shifted = NULL;