fix panic on bad tx sender

Transaction pool would panic on processing a tx with a bad sig.

I can push an example branch with a script showing the panic but it required exporting a `BadSig` function from `chain/types/transaction.go` for convenience (of course, an attacker could just hand craft the rlp tx bytes himself to trigger this). I suppose I should add a `transaction_pool_test.go` with a bunch of handcrafted malicious txs. Here's the fix in the meantime. Have I missed something, or was there no signature check up to now?

Note the panic was on the `GetAccount`, which ultimately triggered https://github.com/ethereum/go-ethereum/blob/develop/trie/trie.go#L299