fix get headers when less than 100 fetched

pkg/geth/blockchain.go

@@ -95,10 +95,12 @@ func (blockChain *BlockChain) getPOWHeaders(blockNumbers []int64) (headers []cor
 	}
 
 	for _, POWHeader := range POWHeaders {
+		if POWHeader.Number != nil {
 			header, _ := blockChain.headerConverter.Convert(&POWHeader, POWHeader.Hash().String())
 
 			headers = append(headers, header)
 		}
+	}
 
 	return headers, err
 }