ui/vnc: fix debug output for invalid audio message

The debug message was cut and pasted from the invalid audio format
case, but the audio message is at bytes 2-3.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

ui/vnc.c

@@ -2551,7 +2551,7 @@ static int protocol_client_msg(VncState *vs, uint8_t *data, size_t len)
                     vs, vs->ioc, vs->as.fmt, vs->as.nchannels, vs->as.freq);
                 break;
             default:
-                VNC_DEBUG("Invalid audio message %d\n", read_u8(data, 4));
+                VNC_DEBUG("Invalid audio message %d\n", read_u8(data, 2));
                 vnc_client_error(vs);
                 break;
             }