Maybe it should not panic in method PrependBytes or AppendBytes

[Hu-Jiang]

In those two function, if we do this

if num < 0 {
	panic("num < 0")
}

And if invoker use num=-1 to invoke those function,
it can not get any error message. So i think maybe

if num < 0 {
	return errors.New("length less than zero")
}

is more reasonable.

gopacket/writer.go

Lines 139 to 178 in 836b571

	func (w *serializeBuffer) PrependBytes(num int) ([]byte, error) {
	if num < 0 {
	panic("num < 0")
	}
	if w.start < num {
	toPrepend := w.prepended
	if toPrepend < num {
	toPrepend = num
	}
	w.prepended += toPrepend
	length := cap(w.data) + toPrepend
	newData := make([]byte, length)
	newStart := w.start + toPrepend
	copy(newData[newStart:], w.data[w.start:])
	w.start = newStart
	w.data = newData[:toPrepend+len(w.data)]
	}
	w.start -= num
	return w.data[w.start : w.start+num], nil
	}
	func (w *serializeBuffer) AppendBytes(num int) ([]byte, error) {
	if num < 0 {
	panic("num < 0")
	}
	initialLength := len(w.data)
	if cap(w.data)-initialLength < num {
	toAppend := w.appended
	if toAppend < num {
	toAppend = num
	}
	w.appended += toAppend
	newData := make([]byte, cap(w.data)+toAppend)
	copy(newData[w.start:], w.data[w.start:])
	w.data = newData[:initialLength]
	}
	// Grow the buffer. We know it'll be under capacity given above.
	w.data = w.data[:initialLength+num]
	return w.data[initialLength:], nil
	}

[notti]

@gconnell is there a reason this is a panic and not an error?