tdf#102688: let's purge it here just in case...

In theory it would still leak without this if the map mode is set
multiple times.

Change-Id: Ie862f4d59e45b92ff75dffb787ef256337f47f84
Reviewed-on: https://gerrit.libreoffice.org/33602Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Kohei Yoshida <libreoffice@kohei.us>

editeng/source/editeng/impedit2.cxx

@@ -208,6 +208,7 @@ void ImpEditEngine::SetRefMapMode( const MapMode& rMapMode )
     if ( GetRefDevice()->GetMapMode() == rMapMode )
         return;
 
+    mpOwnDev.disposeAndClear();
     mpOwnDev = VclPtr<VirtualDevice>::Create();
     pRefDev = mpOwnDev;
     pRefDev->SetMapMode( MapUnit::MapTwip );