Fix race condition in closing a relay

This commit fixes a situration when relay can be reset before all
waiting goroutines are finished. For example, we terminate processing
based on some event: socket error etc. So, error happens and context is
cancelled. After that a main relay goroutine starts to wait. Meanwhile a
second goroutine reaches deferred function and set wg to done. It means
that main goroutine can continue.

In this case this is really possible that we can start resetting before
transmit goroutine really exits.

A correct solution is to always do wg.Done() as a first deferred thing
on entering to a function. In that case we do not need reordering and so
on.

mtglib/internal/relay/relay.go

 
 func (r *Relay) transmit(src io.ReadCloser, dst io.WriteCloser,
 	buffer []byte, direction string, wg *sync.WaitGroup) {
+	defer wg.Done()
+
 	defer func() {
+		r.ctxCancel()
 		src.Close()
 		dst.Close()
-		wg.Done()
-		r.ctxCancel()
 	}()
 
 	if _, err := io.CopyBuffer(dst, src, buffer); err != nil {
 }
 
 func (r *Relay) runObserver(one, another io.Closer, wg *sync.WaitGroup) {
+	defer wg.Done()
+
 	ticker := time.NewTicker(time.Second)
 
 	defer func() {
 		case <-ticker.C:
 		default:
 		}
-
-		wg.Done()
 	}()
 
 	lastTickAt := time.Now()