z parameter expansion flag is broken?

[Yuya Amemiya <ghostrevery@xxxxxxxxx>]

Why does z parameter expansion flag remove leading '-' from result?

in zsh 5.4.1

```
$ echo ${(z):--a}
a
$ echo ${(q):--a}
-a
```

in zsh 5.3.1

```
$ echo ${(z):--a}
-a
$ echo ${(q):--a}
-a
```

regards

--
Yuya Amemiya <ghostrevery@xxxxxxxxx>