Operations with salt fail

_mkpasswd v. 5.6.6_

```
MKPASSWD(1)

 -S STRING, --salt=STRING
  Use the STRING as salt. If it begins with $ then it will be passed straight to crypt(3) without any checks.
```

```
$ mkpasswd -m yescrypt -S " "
Wrong salt length: 1 byte when 0 expected.
$ mkpasswd -m yescrypt -S $" "
Wrong salt length: 1 byte when 0 expected.
```
Should be effective.