ima: fix reference leak in asymmetric_verify()

Don't leak a reference to the key if its algorithm is unknown.

Fixes: 947d7059 ("ima: Support EC keys for signature verification")
Cc: <stable@vger.kernel.org> # v5.13+
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>