Fix: Node: Integer overflow in GPU nodetree evaluation

This was caused by `stack_index` being stored as a `short`
even though the stack `index` inside `ntree_exec_begin` was
`int`.

Rel #146705

Pull Request: https://projects.blender.org/blender/blender/pulls/146733

source/blender/makesdna/DNA_node_types.h

@@ -156,14 +156,14 @@ typedef struct bNodeSocket {
   void *default_value;
 
   /** Local stack index for "node_exec". */
-  short stack_index;
+  int stack_index;
   char display_shape;
 
   /* #AttrDomain used when the geometry nodes modifier creates an attribute for a group
    * output. */
   char attribute_domain;
 
-  char _pad[4];
+  char _pad[2];
 
   /** Custom dynamic defined label. */
   char label[/*MAX_NAME*/ 64];