Fix a bug in SUMSQ

The same one as was in SumMXMY2, with IterateResult.

Change-Id: I18a5993493ca7514848baad25232f4c254c1e044
Reviewed-on: https://gerrit.libreoffice.org/18765Reviewed-by: Łukasz Hryniuk <lukasz.hryniuk@wp.pl>
Tested-by: Łukasz Hryniuk <lukasz.hryniuk@wp.pl>

sc/source/core/tool/interpr6.cxx

@@ -408,6 +408,7 @@ void IterateMatrix(
         case ifSUMSQ:
         {
             ScMatrix::IterateResult aRes = pMat->SumSquare(bTextAsZero);
+            fRes = aRes.mfFirst;
             fRes += aRes.mfRest;
             rCount += aRes.mnCount;
         }