tdf#61228: Wrong function names in Formula Writer are considered as SUM

Unknown names make the formula fail
PythonTest_sw_python, especially set_expression is ok
Variables are taken into account

Thanks to Mike Kaganski for his help on this patch.

Change-Id: Ia6f9c54d90ce88138fd9c9df9422b34ce8223ca2
Reviewed-on: https://gerrit.libreoffice.org/20122Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Mike Kaganski <mikekaganski@hotmail.com>

sw/source/core/bastyp/calc.cxx

@@ -1317,15 +1317,20 @@ SwSbxValue SwCalc::Prim()
         break;
 
     case CALC_NAME:
-        if( GetToken() == CALC_ASSIGN )
+        switch(SwCalcOper nOper = GetToken())
         {
-            SwCalcExp* n = VarInsert( aVarName );
+            case CALC_ASSIGN:
+            {
+                SwCalcExp* n = VarInsert(aVarName);
                 GetToken();
                 nErg = n->nValue = Expr();
             }
+            break;
+            default:
+                nErg = VarLook(aVarName)->nValue;
+                if (nErg.IsVoidValue() && (nOper == CALC_LP))
+                    eError = CALC_SYNTAX;
                 else
-        {
-            nErg = VarLook( aVarName )->nValue;
                     bChkPow = true;
         }
         break;