message: Check that BlockNumber is non-nil in the receipt validation
diff: @@ -30,6 +30,9 @@ func makeReceiptsFn(block eth.BlockID, receiptHash common.Hash) func(txHashes []
  if r.TransactionIndex != uint(i) {
  return nil, fmt.Errorf("receipt %d has unexpected tx index %d", i, r.TransactionIndex)
  }
  + if r.BlockNumber == nil {
  + return nil, fmt.Errorf("receipt %d has unexpected nil block number, expected %d", i, block.Number)
  + }
  if r.BlockNumber.Uint64() != block.Number {
  return nil, fmt.Errorf("receipt %d has unexpected block number %d, expected %d", i, r.BlockNumber, block.Number)
  }
  