sound: avoid endless loop

'sound play 1 100000' results in an endless loop on the sandbox.

If the frequency exceeds half the sampling rate, zero out the output
buffer.

Fixes: 511ed5fdd389 ("SOUND: SAMSUNG: Add I2S driver")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/sound/sound.c b/drivers/sound/sound.c
index 041dfdc..c0fc50c 100644
--- a/drivers/sound/sound.c
+++ b/drivers/sound/sound.c
@@ -15,7 +15,10 @@
 	const int period = freq ? sample_rate / freq : 0;
 	const int half = period / 2;
 
-	assert(freq);
+	if (!half) {
+		memset(data, 0, size);
+		return;
+	}
 
 	/* Make sure we don't overflow our buffer */
 	if (size % 2)