Unspendable keys in descriptors

[andrewtoth]

@andrewkozlik

A potential risk of not hashing is that a malicious participant could craft the chain code in their XPUB so that it cancels out with the other participants to force the result to a desired value. However, such a malicious participant could just as well publish the dummy XPUB to achieve the same goal

I think publishing the dummy XPUB out of band to interested parties is not exactly the same goal as making the dummy XPUB obvious to any observer of the transaction.