check for 0 length strings in pubkey parser.

We check length later, but we assumed it was always 1 bytes long. Not
always the case. I'm a little depressed that this bug was there.

pubkey.go

@@ -6,6 +6,7 @@ package btcec
 
 import (
 	"crypto/ecdsa"
+	"errors"
 	"fmt"
 	"math/big"
 )
@@ -53,6 +54,10 @@ func ParsePubKey(pubKeyStr []byte, curve *KoblitzCurve) (key *ecdsa.PublicKey, e
 	pubkey := ecdsa.PublicKey{}
 	pubkey.Curve = curve
 
+	if len(pubKeyStr) == 0 {
+		return nil, errors.New("pubkey string is empty")
+	}
+
 	format := pubKeyStr[0]
 	ybit := (format & 0x1) == 0x1
 	format &= ^byte(0x1)