Improve the example getpass() implementation to show an error return

Also, modernize the code, so that it isn't trying to store a size_t
into an int, and then check the int's sign. :/

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6271)