index out of range panic in ErrorCode()

[juhaszp-uhu]

I've got a runtime panic while running some code that was essentially identical to the example server in this repo:

panic: runtime error: index out of range [3] with length 0

goroutine 10 [running]:
github.com/CyCoreSystems/audiosocket.Message.ErrorCode(...)
        /home/pjuhasz/pkg/mod/github.com/!cy!core!systems/audiosocket@v0.2.0/audiosocket.go:75

My analysis:

For some reason audiosocket.NextMessage() returned a zero length message. I don't know if there was an error, unfortunately I haven't checked it, but the example code doesn't either.
Then that empty message is passed to m.Kind(), which returns KindError. This is problem number one, that a legitimate packet type is conflated with something that is more like a condition on the receiving site.
Then, believing that the message is valid, it is passed to m.ErrorCode(), which then panics, because it blindly accesses m[3] without making sure that the message is long enough for that.

Suggestions:

• check the length in ErrorCode() - I think this is easy and harmless
• if possible, introduce a new error code to signify that the packet is invalid, or signal the error in some other way.

[vcidst]

This is marked with a //FIXME comment already in the code,

audiosocket/audiosocket.go

Lines 68 to 76 in 807c189

	// ErrorCode returns the coded error of the message, if present
	func (m Message) ErrorCode() ErrorCode {
	if m.Kind() != KindError {
	return ErrNone
	}
	// FIXME: TBD
	return ErrorCode(m[3])
	}