validate: don't chase non-sensical signers
When signer name isn't a prefix of owner, the signature does not make sense and it's no use trying to use that signer name in any way.
We generally don't force queries on every level of the path, so this signer confusion could "introduce SERVFAILs" if we skip over a transition to insecure.
Fixes: #587 (closed)
Edited by Vladimír Čunát