add checks for grfX handlers to prevent divide-by-zero with slimproto (…

…#424)

Thank you!

components/squeezelite/displayer.c

@@ -687,6 +687,7 @@ void draw_VU(struct GDS_Device * display, int level, int x, int y, int width, bo
 static void grfe_handler( u8_t *data, int len) {
  struct grfe_packet *pkt = (struct grfe_packet*) data; 
 
+ GDS_CHECK_FOR_DEVICE(display,return); 
  // we don't support transition, simply claim we're done
  if (pkt->transition != 'c') {
  LOG_INFO("Transition %c requested with offset %hu, param %d", pkt->transition, pkt->offset, pkt->param);
@@ -773,7 +774,8 @@ static void grfs_handler(u8_t *data, int len) {
  htons(pkt->width), // last column of animation that contains a "full" screen
  htons(pkt->offset) // offset if multiple packets are sent
  );
-
+
+ GDS_CHECK_FOR_DEVICE(display,return); 
  // new grfs frame, build scroller info
  if (!offset) { 
  // use the display as a general lock
@@ -820,6 +822,7 @@ static void grfg_handler(u8_t *data, int len) {
 
  LOG_DEBUG("gfrg s:%hu w:%hu (len:%u)", htons(pkt->screen), htons(pkt->width), len);
 
+ GDS_CHECK_FOR_DEVICE(display,return); 
  // full screen artwork or for small screen, visu has priority when full screen 
  if (((visu.mode & VISU_ESP32) && !visu.col && visu.row < displayer.height) || artwork.full) {
  return;
@@ -864,6 +867,7 @@ static void grfa_handler(u8_t *data, int len) {
  int offset = htonl(pkt->offset);
  int length = htonl(pkt->length);
 
+ GDS_CHECK_FOR_DEVICE(display,return); 
  // when using full screen visualizer on small screen there is a brief overlay 
  artwork.enable = (length != 0);