Call Button_Quit and Button_Page with correct enum

These functions are called explicitly in a few places and need to be
handed the corresponding enum value to correctly get the button value.
Since they were previously called with -1, they would cause segfaults.
These seemed to happen when restoring backups or loading images to both
pages.

src/loadsave.c

@@ -1588,7 +1588,7 @@ if (Create_lock_file(Config_directory))
     Compute_limits();
     Compute_paintbrush_coordinates();
     if (Backups_main)
-      Button_Page(-1);
+      Button_Page(BUTTON_PAGE);
   }
   restored_main = Process_backups(&Backups_main);