Update documentation for PEM callback: error is now -1.

In previous versions of OpenSSL, the documentation for PEM_read_*
said:

   The callback B<must> return the number of characters in the
   passphrase or 0 if an error occurred.

But since c82c3462

, 0 is now treated as a non-error
return value.  Applications that want to indicate an error need to
return -1 instead.

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 bbbf752a)