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)

(cherry picked from commit c8c250333cd254ab3f4d709ebc5ed86a7c065721)
(cherry picked from commit 50d06d1c7d2682b0042e921a76beb509d7ea68e1)