Have OSSL_PARAM_allocate_from_text() fail on odd number of hex digits

Have OSSL_PARAM_allocate_from_text() fail on odd number of hex digits

The failure would be caught later on, so this went unnoticed, until someone
tried with just one hex digit, which was simply ignored.

Fixes #23373

Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/23374)