Some panics to be handled on adversarial input

No need to merge this. Probably we should start an `Adversary` folder in the `Tests` directory and begin to standardize this sort of thing. But here I'm pointing out that
- NewValueFromBytes panics on some forms of bad rlp 
- ValidateTransaction panics on a bad signature (this is fixed here, by making the sig check explicit: https://github.com/ethereum/go-ethereum/pull/195)

These are important because they are called more or less directly after receiving raw bytes from peers.