fdo#58539: Check for mxValueLink being NULL before dereferencing.

Or else Calc would crash while loading the xls document from
gnome519788.

As an aside, this xls document appears to be corrupt. Trying to
open it in Excel (XP and 2007) causes Excel to offer to repair it,
while fails in both versions.  Excel XP is somehow able to open it
with some content preserved. No such luck with Excel 2007.

Change-Id: I04616a4c926862461a2efdd99ccabe36122d6825

sc/source/filter/excel/xichart.cxx

@@ -2057,7 +2057,7 @@ Reference< XDataSeries > XclImpChSeries::CreateDataSeries() const
         aSeriesProp.SetBoolProperty( EXC_CHPROP_VARYCOLORSBY, rTypeInfo.meTypeCateg == EXC_CHTYPECATEG_PIE );
 #endif
         // #i91271# always set area formatting for every point in pie/doughnut charts
-        if( mxSeriesFmt && ((bVarPointFmt && mxSeriesFmt->IsAutoArea()) || (rTypeInfo.meTypeCateg == EXC_CHTYPECATEG_PIE)) )
+        if (mxSeriesFmt && mxValueLink && ((bVarPointFmt && mxSeriesFmt->IsAutoArea()) || (rTypeInfo.meTypeCateg == EXC_CHTYPECATEG_PIE)))
         {
             for( sal_uInt16 nPointIdx = 0, nPointCount = mxValueLink->GetCellCount(); nPointIdx < nPointCount; ++nPointIdx )
             {