tdf#131380 crash on loading specific xlsx document

mpPerIdx is null, because bEDS is true, assume 0 in this case

Change-Id: I44c2433b0de57a28cece6b9b01ba543bdc3cb8d3
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/91145
Tested-by: Jenkins
Reviewed-by: Eike Rathke <erack@redhat.com>
diff --git a/sc/source/core/tool/interpr8.cxx b/sc/source/core/tool/interpr8.cxx
index 278e253..2e15c14 100644
--- a/sc/source/core/tool/interpr8.cxx
+++ b/sc/source/core/tool/interpr8.cxx
@@ -1002,17 +1002,18 @@ void ScETSForecastCalculation::GetETSPredictionIntervals( const ScMatrixRef& rTM
        // fill array with forecasts, with RandDev() added to xScenRange
        if ( bAdditive )
        {
            double nPIdx = !bEDS ? mpPerIdx[mnCount - mnSmplInPrd] : 0.0;
            // calculation based on additive model
            xScenRange[ 0 ] = mpBase[ mnCount - 1 ] + mpTrend[ mnCount - 1 ] +
                              mpPerIdx[ mnCount - mnSmplInPrd ] +
                              nPIdx +
                              RandDev();
            aPredictions[ 0 ][ k ] = xScenRange[ 0 ];
            xScenBase[ 0 ] = mfAlpha * ( xScenRange[ 0 ] - mpPerIdx[ mnCount - mnSmplInPrd ] ) +
            xScenBase[ 0 ] = mfAlpha * ( xScenRange[ 0 ] - nPIdx ) +
                             ( 1 - mfAlpha ) * ( mpBase[ mnCount - 1 ] + mpTrend[ mnCount - 1 ] );
            xScenTrend[ 0 ] = mfGamma * ( xScenBase[ 0 ] - mpBase[ mnCount - 1 ] ) +
                              ( 1 - mfGamma ) * mpTrend[ mnCount - 1 ];
            xScenPerIdx[ 0 ] = mfBeta * ( xScenRange[ 0 ] - xScenBase[ 0 ] ) +
                               ( 1 - mfBeta ) * mpPerIdx[ mnCount - mnSmplInPrd ];
                               ( 1 - mfBeta ) * nPIdx;
            for ( SCSIZE i = 1; i < nSize; i++ )
            {
                double fPerIdx;