No action if password exceeds PAM_MAX_RESP_SIZE

[kpp]

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https://github.com/memorysafety/sudo-rs/blob/55e09d74d43a1bb185893cb4a3f9e483e0a2d052/lib/sudo-pam/src/rpassword.rs#L81

That's an unlikely situation but there is no actions taken if the password is longer than PAM_MAX_RESP_SIZE (512) bytes.