Resolves: tdf#109856 undo 'insert multiple tabs after' doesn't work

Change-Id: I534536358b2a949667c1c258bf27e13763c0de57
Reviewed-on: https://gerrit.libreoffice.org/40620Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

sc/source/core/data/document.cxx

@@ -786,7 +786,7 @@ void ScDocument::ClearTabs()
 bool ScDocument::DeleteTabs( SCTAB nTab, SCTAB nSheets )
 {
     bool bValid = false;
-    if (ValidTab(nTab) && (nTab + nSheets) < static_cast<SCTAB>(maTabs.size()))
+    if (ValidTab(nTab) && (nTab + nSheets) <= static_cast<SCTAB>(maTabs.size()))
     {
         if (maTabs[nTab])
         {