Fix AudioOutputUSB handling of null data

teensy3/usb_audio.cpp

 {
 	audio_block_t *left, *right;
 
-	left = receiveReadOnly(0); // input 0 = left channel
-	right = receiveReadOnly(1); // input 1 = right channel
+	// TODO: we shouldn't be writing to these......
+	//left = receiveReadOnly(0); // input 0 = left channel
+	//right = receiveReadOnly(1); // input 1 = right channel
+	left = receiveWritable(0); // input 0 = left channel
+	right = receiveWritable(1); // input 1 = right channel
 	if (usb_audio_transmit_setting == 0) {
 		if (left) release(left);
 		if (right) release(right);
 		return;
 	}
 	if (left == NULL) {
-		if (right == NULL) return;
-		right->ref_count++;
-		left = right;
-	} else if (right == NULL) {
-		left->ref_count++;
-		right = left;
+		left = allocate();
+		if (left == NULL) {
+			if (right) release(right);
+			return;
+		}
+		memset(left->data, 0, sizeof(left->data));
+	}
+	if (right == NULL) {
+		right = allocate();
+		if (right == NULL) {
+			release(left);
+			return;
+		}
+		memset(right->data, 0, sizeof(right->data));
 	}
 	__disable_irq();
 	if (left_1st == NULL) {