vcl: handle nDPI == 0 in ImplPixelToLogic()

To avoid the floating point exception that can be seen sometimes on the
Jenkins_Linux tinderbox during CppunitTest_sw_uiwriter.

Change-Id: I270bfbe9c8fbb4cb3a86f45b5f4c3afb32213b9e

vcl/source/outdev/map.cxx

@@ -402,7 +402,7 @@ static long ImplPixelToLogic( long n, long nDPI, long nMapNum, long nMapDenom,
 {
     // To "use" it...
    (void) nThres;
-   if (nMapNum == 0)
+   if (nMapNum == 0 || nDPI == 0)
    {
        return 0;
    }