problem: good-will assumption for forks with required hash

solution: HeaderCheck check for new ErrHashEmpty error,

where that error is ignored by eth/handler.go.
This is presented as a possible alternative/+addition to @sorpaas changed in ethereumproject#313

Rel ethereumproject#309

core/config.go

@@ -46,6 +46,8 @@ var (
 
 	ErrHashKnownBad  = errors.New("known bad hash")
 	ErrHashKnownFork = validateError("known fork hash mismatch")
+
+	ErrHashEmpty = errors.New("empty hash")
 )
 
 // SufficientChainConfig holds necessary data for externalizing a given blockchain configuration.
@@ -333,6 +335,10 @@ func (c *ChainConfig) GetFeature(num *big.Int, id string) (*ForkFeature, *Fork,
 }
 
 func (c *ChainConfig) HeaderCheck(h *types.Header) error {
+	if (h == &types.Header{}) {
+		return ErrHashEmpty
+	}
+
 	for _, fork := range c.Forks {
 		if fork.Block.Cmp(h.Number) != 0 {
 			continue

eth/handler.go

@@ -286,7 +286,7 @@ func (pm *ProtocolManager) handle(p *peer) error {
 		}
 		if !fork.RequiredHash.IsEmpty() {
 			// Request the peer's fork block header for extra-dat
-			if err := p.RequestHeadersByNumber(fork.Block.Uint64(), 1, 0, false); err != nil {
+			if err := p.RequestHeadersByNumber(fork.Block.Uint64(), 1, 0, false); err != nil && err != core.ErrHashEmpty {
 				glog.V(logger.Warn).Infof("%v: error requesting headers by number ", p)
 				return err
 			}