Maybe it should not panic in method PrependBytes or AppendBytes

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.

https://github.com/google/gopacket/blob/836b571ec913f74022809b997ad331ebbf1033d9/writer.go#L139-L178