Fix a merge mistake in engne_list_add

Fix a merge mistake in engne_list_add

master version increments the struct_ref early
and needs to decrement the struct_ref on error,
while 3.1 and 3.0 increment the struct_ref later.

Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22115)