i2b_PVK(): Use Encrypt, not Decrypt

i2b_PVK(): Use Encrypt, not Decrypt

We used EVP_EncryptInit_ex() to initialise, but EVP_DecryptUpdate()
and EVP_DecryptFinal_ex() to actually perform encryption.  This worked
long ago, when the Encrypt and Decrypt variants were the same, but
doesn't now (actually haven't for a very long time).

This shows how seldom PVK is actually used.

Fixes #9338

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/10521)