Codefix: Avoid type-casting function pointer with incorrect type. (#12929)

Make `AutolengthProc` take `int` instead of `void *`, avoiding pointer parameters.

src/saveload/saveload.cpp

@@ -1925,7 +1925,7 @@ void SlGlobList(const SaveLoadTable &slt)
  * @param proc The callback procedure that is called
  * @param arg The variable that will be used for the callback procedure
  */
-void SlAutolength(AutolengthProc *proc, void *arg)
+void SlAutolength(AutolengthProc *proc, int arg)
 {
 	assert(_sl.action == SLA_SAVE);