Fix memory leaks in pkread.c demo file

Also make various changes to bring the file into line with current coding
style.

Fixes #8456

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Paul Yang <yang.yang@baishancloud.com>
(Merged from https://github.com/openssl/openssl/pull/8457)

(cherry picked from commit 81cd023f1dc5a8c9094f8e91c1e85e3c9b98a551)