PubKeyFromBytes should return a blank PubKey on failing to parse bytes

[odeke-em]

The current implementation of PubKeyFromBytes could be improved. It currently is

go-crypto/pub_key.go

Lines 16 to 19 in dd20358

	func PubKeyFromBytes(pubKeyBytes []byte) (pubKey PubKey, err error) {
	err = wire.ReadBinaryBytes(pubKeyBytes, &pubKey)
	return
	}

Notice that even if err != nil we took the address of the named return value so set some stuff in it.
The problem though is that if we try to parse a key with invalid bytes and that fails, later on test
pubKey.Empty() will return false which is wrong.

This behavior skewed my fuzzing tests and here is a simple repro

package main

import (
	"log"

	"github.com/tendermint/go-crypto"
)

func main() {
	data := []byte{0x02, 0x03}
	pk, err := crypto.PubKeyFromBytes(data)
	log.Printf("empty: %v\ninner: %#v\n", pk.Empty(), pk.PubKeyInner)
	if err != nil {
		log.Fatalf("err: %v\n", err)
	}
	log.Printf("pk: %#v\n", pk)
}

which gives

$ go run main.go 
2017/10/28 14:05:55 empty: false
inner: crypto.PubKeySecp256k1{0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0}
2017/10/28 14:05:55 err: unexpected EOF
exit status 1

yet it should actually give

2017/10/28 14:06:21 empty: true
inner: <nil>
2017/10/28 14:06:21 err: unexpected EOF
exit status 1