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

Change-Id: I2c9b78f4d1a963bb71abb21068e61e6983ade777
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/150484
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>

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;