core: fix potential issue with header list init

If we're to receive some 500 headers (less than `headerBatchCount`) and quit
before receiving more of them we end up with clean `bc.headerList` that will
be inited going backwards to the `targetHash`, but code path doesn't add add
the `targetHash` itself which it should do in this particular case, otherwise
we end with no genesis block hash in the list.

pkg/core/blockchain.go

@@ -153,6 +153,7 @@ func (bc *Blockchain) init() error {
 				return err
 			}
 			targetHash = genesisBlock.Hash()
+			bc.headerList.Add(targetHash)
 		}
 		headers := make([]*Header, 0)
 
@@ -164,7 +165,6 @@ func (bc *Blockchain) init() error {
 			headers = append(headers, header)
 			hash = header.PrevHash
 		}
-
 		headerSliceReverse(headers)
 		for _, h := range headers {
 			if !h.Verify() {