coverity#735656 Division or modulo by zero

Change-Id: I846f5f10649e2f12bac763c87e88cd55135f3d00

vcl/source/control/lstbox.cxx

@@ -1347,7 +1347,7 @@ Size ListBox::CalcBlockSize( sal_uInt16 nColumns, sal_uInt16 nLines ) const
 
 void ListBox::GetMaxVisColumnsAndLines( sal_uInt16& rnCols, sal_uInt16& rnLines ) const
 {
-    long nCharWidth = GetTextWidth( OUString(static_cast<sal_Unicode>('x')) );
+    float nCharWidth = approximate_char_width();
     if ( !IsDropDownBox() )
     {
         Size aOutSz = mpImplLB->GetMainWindow()->GetOutputSizePixel();