wintab: fix skipping of odd-numbered devices

Fix a regression introduced in 4ce6d10
which causes devices with an odd-numbered zero-based index in the list
to be passed over incorrectly. This might present as yet another "device
does not send pressure" bug for ~50% of devices out there.

This commit also closes off another potential segfault for wintab_devices
lists which have an odd length.

https://bugzilla.gnome.org/show_bug.cgi?id=774699

gdk/win32/gdkdevicemanager-win32.c

@@ -852,10 +852,9 @@ gdk_device_manager_find_wintab_device (GdkDeviceManagerWin32 *device_manager,
   GdkDeviceWintab *device;
   GList *tmp_list;
 
-  for (tmp_list = device_manager->wintab_devices; tmp_list; tmp_list = tmp_list->next)
+  for (tmp_list = device_manager->wintab_devices; tmp_list != NULL; tmp_list = tmp_list->next)
     {
       device = tmp_list->data;
-      tmp_list = tmp_list->next;
 
       if (device->hctx == hctx &&
           device->cursor == cursor)