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

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)