mx_flock: Don't call _flock_free to early

Don't call _flock_free before flock->fname has been initialized.

mx_flock.c

@@ -89,7 +89,7 @@ struct mx_flock *mx_flock(int operation, char *fmt, ...)
 
     if (res == -1) {
        mx_log_err("vasprintf(): %m");
-       _flock_free(lock);
+       free(lock);
        return NULL;
     }