Fix range check bug in FileList::seek() (#13286)

When `count()` returns 0, `pos > (count()-1)` will always yield `true` due to integer underflow.

Marlin/src/lcd/extensible_ui/ui_api.cpp

@@ -688,7 +688,7 @@ namespace ExtUI {
 
   bool FileList::seek(const uint16_t pos, const bool skip_range_check) {
     #if ENABLED(SDSUPPORT)
-      if (!skip_range_check && pos > (count() - 1)) return false;
+      if (!skip_range_check && (pos + 1) > count()) return false;
       const uint16_t nr =
         #if ENABLED(SDCARD_RATHERRECENTFIRST) && DISABLED(SDCARD_SORT_ALPHA)
           count() - 1 -