tdf#89387 related, fix SumSquare bug in SumMXMY2

SumSquare returns IterateResult, which, because of rounding errors, store
first value in mfFirst and rest in mfRest. They should be added to get
the full sum of square of matrices difference.

Change-Id: I9bc435801d94206960b0b4fe4a9149ef1abeb3be
Reviewed-on: https://gerrit.libreoffice.org/17592Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>

sc/source/core/tool/interpr5.cxx

@@ -1791,7 +1791,7 @@ void ScInterpreter::ScSumXMY2()
     else
     {
         ScMatrix::IterateResult aRes = pResMat->SumSquare(false);
-        double fSum = aRes.mfRest;
+        double fSum = aRes.mfFirst + aRes.mfRest;
         PushDouble(fSum);
     }
 }