Prevent out-of-bounds access, tdf#114228 related

Such document access should be validated, specifically if it's
called from accessibility.

Change-Id: I67fa14c7fb1bf3885fc009428b981149f3f448fb
Reviewed-on: https://gerrit.libreoffice.org/45978Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>

sc/source/core/data/document.cxx

@@ -3681,7 +3681,7 @@ sal_uInt32 ScDocument::GetNumberFormat( const ScRange& rRange ) const
     SCCOL nCol1 = rRange.aStart.Col(), nCol2 = rRange.aEnd.Col();
     SCROW nRow1 = rRange.aStart.Row(), nRow2 = rRange.aEnd.Row();
 
-    if (!ValidTab(nTab1) || !ValidTab(nTab2) || !maTabs[nTab1] || !maTabs[nTab2])
+    if (!TableExists(nTab1) || !TableExists(nTab2))
         return 0;
 
     sal_uInt32 nFormat = 0;