reader returns eof is header is missing bytes but returns error if record is missing bytes

[morzaria-stripe]

If header is <4 bytes it will treat it as EOF https://github.com/colinmarc/sequencefile/blob/master/reader.go#L120-L121
but if the record is missing bytes then it will treat it as error https://github.com/colinmarc/sequencefile/blob/master/reader.go#L145-L148.