kmac_prov.c.in: avoid resource leak on kmac_new_decoder fail in kmac_fetch_new

kmac_prov.c.in: avoid resource leak on kmac_new_decoder fail in kmac_fetch_new

kctx was not freed in a case of kmac_new_decoder failure;  consolidate
all the error paths under the "err:" label and jump to it on kmac_new_decoder()
returning 0.

Fixes: d5efc853796b "kmac: avoid using ossl_prov_digest_load_from_params()"
Resolves: https://github.com/openssl/project/issues/1419
Resolves: https://scan5.scan.coverity.com/#/project-view/65248/10222?selectedIssue=1453634
Signed-off-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Saša Nedvědický <sashan@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Neil Horman <nhorman@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/28516)