Check for NULL cleanup function before using it in encoder_process

Check for NULL cleanup function before using it in encoder_process

encoder_process assumes a cleanup function has been set in the currently
in-use encoder during processing, which can lead to segfaults if said
function hasn't been set

Add a NULL check for this condition, returning -1 if it is not set

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/23069)