Resolves: tdf#146068 don't crash when MA(central) lacks data

Change-Id: I2c9b78f4d1a963bb71abb21068e61e6983ade777
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/150501
Tested-by: Jenkins
Reviewed-by: Michael Stahl <michael.stahl@allotropia.de>
diff --git a/chart2/source/tools/MovingAverageRegressionCurveCalculator.cxx b/chart2/source/tools/MovingAverageRegressionCurveCalculator.cxx
index 6236f73..6726d9d 100644
--- a/chart2/source/tools/MovingAverageRegressionCurveCalculator.cxx
+++ b/chart2/source/tools/MovingAverageRegressionCurveCalculator.cxx
@@ -84,6 +84,8 @@ void MovingAverageRegressionCurveCalculator::calculateValuesCentral(
    RegressionCalculationHelper::tDoubleVectorPair aValues)
{
    const size_t aSize = aValues.first.size();
    if (aSize == 0)
        return;
    for (size_t i = mPeriod - 1; i < aSize; ++i)
    {
        double yAvg = 0.0;