lmdbscan: Scanner.Scan panics if lmdbscan.New() failed.

[bmatsuo]

If lmdbscan.New() fails then the error will be wiped out when the scanner's Scan method will panic attempting to access the underlying cursor. This is counter to the goals of the lmdbscan package, where ideally errors are checked for once at the end of the transaction.

scanner.New(txn, 1234)
defer scanner.Close()
for scanner.Scan() { // panic!
  log.Printf("k=%q v=%q", scanner.Key(), scanner.Val())
}
return scanner.Err()