Set the terminal tilegrid NULL after free

Without this, a double free can occur when a display (and terminal)
is released and then a crash occurs. Upon a second release,
different memory is released (sometimes the heap). When this is
followed by an allocation for the flash cache, the cache can
overwrite the active heap causing crashes.

Fixes #1667

supervisor/shared/display.c

@@ -81,6 +81,7 @@ void supervisor_start_terminal(uint16_t width_px, uint16_t height_px) {
 void supervisor_stop_terminal(void) {
     if (tilegrid_tiles != NULL) {
         free_memory(tilegrid_tiles);
+        tilegrid_tiles = NULL;
         supervisor_terminal_text_grid.inline_tiles = false;
         supervisor_terminal_text_grid.tiles = NULL;
     }