enhancement: improve stability of the methods of buf.Buffer struct

[leungyauming]

Relevant Issue: #622

// Byte returns the bytes at index.
func (b *Buffer) Byte(index int32) byte {
	return b.v[b.start+index]
}

// SetByte sets the byte value at index.
func (b *Buffer) SetByte(index int32, value byte) {
	b.v[b.start+index] = value
}

// BytesRange returns a slice of this buffer with given from and to boundary.
func (b *Buffer) BytesRange(from, to int32) []byte {
	if from < 0 {
		from += b.Len()
	}
	if to < 0 {
		to += b.Len()
	}
	return b.v[b.start+from : b.start+to]
}

// BytesFrom returns a slice of this Buffer starting from the given position.
func (b *Buffer) BytesFrom(from int32) []byte {
	if from < 0 {
		from += b.Len()
	}
	return b.v[b.start+from : b.end]
}

// BytesTo returns a slice of this Buffer from start to the given position.
func (b *Buffer) BytesTo(to int32) []byte {
	if to < 0 {
		to += b.Len()
	}
	return b.v[b.start : b.start+to]
}

The above methods will have a very high change in leading to the following panic:
panic: runtime error: index out of range [x] with length x

Any thoughts on improving the above issue?
I may contribute code to it but I want to ensure the direction before I start working on it.