lokit: double check for disposed windows & fix leak.

lokit: double check for disposed windows & fix leak.

it seems we can get windows disposed before we get to the emission.

<segv>
vcl::Window::IsTracking() const vcl/source/window/window2.cxx:341
(anonymous namespace)::LOKPostAsyncEvent(void*, void*) sfx2/source/view/lokhelper.cxx:725
LokChartHelper::postMouseEvent(int, int, int, int, int, int, double, double) include/rtl/ref.hxx:112
SwXTextDocument::postMouseEvent(int, int, int, int, int, int) sw/source/uibase/uno/unotxdoc.cxx:3561

Change-Id: I93aea931dad1e7f43d3d610568424c53d2b22fbc
Signed-off-by: Michael Meeks <michael.meeks@collabora.com>
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/127907
(cherry picked from commit 7ba99b8d6da07c2133c644de4d1a3613a9fa5a68)
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/129839
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>