Reset WelfordRunner in ScDPAggData::Reset()

Change-Id: I170b6bc5557f6d0d982751ace3dccc94c6778808
Reviewed-on: https://gerrit.libreoffice.org/65048Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Jenkins
(cherry picked from commit ec3ab1c6)
Reviewed-on: https://gerrit.libreoffice.org/65057

sc/source/core/data/dptabres.cxx

@@ -665,6 +665,7 @@ ScDPAggData* ScDPAggData::GetChild()
 
 void ScDPAggData::Reset()
 {
+    maWelford = WelfordRunner();
     fVal = 0.0;
     fAux = 0.0;
     nCount = SC_DPAGG_EMPTY;