No validation of psbts on client side

[f321x]

The current client does no detailed validation of the psbts it gets presented to sign. It would be good practice to perform some checks on the transactions before signing it and returning the signature to the coordinator.
The coordinator is trusted anyways, so malicious behaviour is not assumed, but bugs could happen, for example use of a different change address than the one submitted by the client, which could be detected by client side validation.