Fix compiling warnings in example code

The example code in EVP_DigestInit.pod generates warnings if users try
to compile it.

[skip ci]

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7362)

(cherry picked from commit 19ac1bf2de07214ee7ee6d2e118fa3aa8e5850f3)