Protection against division by zero when scrolling on an empty thumbt…

…able

There are different ways to check if the thumbtable is empty (for example,
whether the thumbnail list is NULL). But we check whether the thumbnail size
is zero as this is exactly what we are interested in, because later we would
have to divide by this value, so we protect ourselves from division by zero.

src/dtgtk/thumbtable.c

@@ -1,6 +1,6 @@
 /*
     This file is part of darktable,
-    Copyright (C) 2024 darktable developers.
+    Copyright (C) 2024-2025 darktable developers.
 
     darktable is free software: you can redistribute it and/or modify
     it under the terms of the GNU General Public License as published by
@@ -1046,9 +1046,15 @@ void dt_thumbtable_zoom_changed(dt_thumbtable_t *table,
 
 static gboolean _event_scroll_compressed(gpointer user_data)
 {
-  if(!user_data) return FALSE;
+  if(!user_data)
+    return FALSE;
+
   dt_thumbtable_t *table = user_data;
 
+  // Thumbtable is empty, nothing to scroll
+  if(table->thumb_size == 0)
+    return FALSE;
+
   if(table->scroll_value != 0)
   {
     float delta = table->scroll_value;