Pi Firmware: fix a crash with OSD when DOUBLE_BUFFER undefined

Change-Id: I2c308450bbcec31234bdecbe4bb4985bb29aebf1

src/osd.c

@@ -185,7 +185,9 @@ void osd_clear() {
    if (active) {
       memset(buffer, 0, sizeof(buffer));
       osd_update((uint32_t *)fb, pitch);
+#ifdef DOUBLE_BUFFER
       osd_update((uint32_t *)(fb + SCREEN_HEIGHT * pitch), pitch);
+#endif
       active = 0;
       RPI_SetGpioValue(LED1_PIN, active);
       update_palette();
@@ -206,7 +208,9 @@ void osd_set(int line, int attr, char *text) {
    }
    strncpy(buffer + line * LINELEN, text, len);
    osd_update((uint32_t *)fb, pitch);
+#ifdef DOUBLE_BUFFER
    osd_update((uint32_t *)(fb + SCREEN_HEIGHT * pitch), pitch);
+#endif
 }