Fix out-of-range access. (#2082)

Happens when converting quality in subscibed settings to layer.
Looks like it can happen only if the provided quality is OFF.
Don't know of any client that does that. Anyhow, prevent out-of-range
access which causea a panic.

pkg/sfu/receiver.go

@@ -539,7 +539,7 @@ func (w *WebRTCReceiver) getBufferLocked(layer int32) *buffer.Buffer {
 		layer = 0
 	}
 
-	if int(layer) >= len(w.buffers) {
+	if layer < 0 || int(layer) >= len(w.buffers) {
 		return nil
 	}