tdf#121198: fix crash in SdrPage::GetPageNum()

Regression from 9348b322

Change-Id: Ibf9d6b65f0dbcc3a751e3ac5f3dbc28d908d6c25
Reviewed-on: https://gerrit.libreoffice.org/62952Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
Tested-by: Jenkins
Reviewed-by: Julien Nabet <serval2412@yahoo.fr>

sd/source/ui/slidesorter/controller/SlsInsertionIndicatorHandler.cxx

@@ -220,7 +220,7 @@ bool InsertionIndicatorHandler::IsInsertionTrivial (
     // to check that the insertion position is not directly in front or
     // directly behind the selection and thus moving the selection there
     // would not change the model.
-    return nInsertionIndex >= nFirstIndex && nInsertionIndex < nLastIndex;
+    return nInsertionIndex >= nFirstIndex && nInsertionIndex <= (nLastIndex+1);
 }
 
 bool InsertionIndicatorHandler::IsInsertionTrivial (const sal_Int8 nDndAction)