Staging: speakup: don't die if accessing sysfs without synth

Setting a 'silent' parameter without a synth would crash the kernel.
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/speakup/synth.c

@@ -212,6 +212,9 @@ void synth_start(void)
 
 void spk_do_flush(void)
 {
+	if (!synth)
+		return;
+
 	speakup_info.flushing = 1;
 	synth_buffer_clear();
 	if (synth->alive) {