ask-password-api: fix error handling on invalid unicode character

The integer overflow happens when utf8_encoded_valid_unichar() returns an error
code. The error code is a negative number: -22. This overflows when it is
assigned to `z` (type `size_t`). This can cause an infinite loop if the value
of `q` is 22 or larger.

To reproduce the bug, you need to run `systemd-ask-password` and enter an
invalid unicode character, followed by a backspace character.

GHSL-2021-052