fix ReadFrom generate corrupted bitset when reader incompletely fills buf

[mingmingtsao]

io.Read does not guarantee read len(p) bytes.

Read reads up to len(p) bytes into p. It returns the number of bytes read (0 <= n <= len(p)) and any error encountered. Even if Read returns n < len(p), it may use all of p as scratch space during the call. If some data is available but not len(p) bytes, Read conventionally returns what is available instead of waiting for more.

[thanhpk]

I think this function is getting complicated more than it should be.

I propose a version that is less performant but much simpler.
No buffio or limitedreader hidden stuffs, just plain binary.Read.

// ReadFrom reads a BitSet from a stream written using WriteTo
func (b *BitSet) ReadFrom(stream io.Reader) (int64, error) {
	var length uint64

	// Read length first
	err := binary.Read(stream, binaryOrder, &length)
	if err != nil {
		if err == io.EOF {
			err = io.ErrUnexpectedEOF
		}
		return 0, err
	}
	newset := New(uint(length))

	if uint64(newset.length) != length {
		return 0, errors.New("unmarshalling error: type mismatch")
	}

	nWords := uint64(wordsNeeded(uint(length)))
	for i := uint64(0); i < nWords; i++ {
		var item uint64
		if err := binary.Read(stream, binaryOrder, &item); err != nil {
			if err == io.EOF {
				err = io.ErrUnexpectedEOF
			}
			return 0, err
		}
		newset.set[i] = item
	}

	*b = *newset
	return int64(b.BinaryStorageSize()), nil
}

[lemire]

I am merging this and re-releasing. Thanks.