bound the configured audio buffer count

Buffers had a lower bound but no upper one, and it is allocated per speaking
user twice over: as a queue of decoded frames and as OpenAL playback
buffers. Each buffer holds up to one maximum sized frame, so a large value
multiplied by a populated channel is a substantial amount of memory.

Cap it, and reject the flag up front so the failure names the flag rather
than surfacing later as a dial error.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

main.go

@@ -105,6 +105,10 @@ func main() {
 	if err != nil {
 		handle_raw_error(err)
 	}
+	if *buffers <= 0 || *buffers > gumble.MaximumBuffers {
+		handle_raw_error(fmt.Errorf("buffers must be between 1 and %d, got %d",
+			gumble.MaximumBuffers, *buffers))
+	}
 
 	// Set up logging
 	var level barnlog.Level