bugfix in cmac calculation example

The example never executes code inside of the while loop, as read()
returns bigger number than 0. Thus the end result is wrong.

CLA: trivial

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/11143)