Fix sample code

Fix memory leak in sample encryption code and check return value of
fopen.

CLA: trivial

Signed-off-by: Greg Zaverucha <gregz@microsoft.com>

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3790)