Missing pre-image checks for shreds in keyguard / keyguard finishing touches

[ripatel-fd]

fd_keyguard does not verify that the pre-image of Merkle shreds is valid and unambiguous with other inputs.

The shred tile could thus sign

• gossip pings
• repair pings
• any arbitrary 32 byte message

This is currently not a very severe issue, but it defeats the purpose of the ambiguity checks.