SfxInPlaceClient: avoid crash when chart deleted by other view

Be sure m_pClient still is valid. In case when one view
was in the chart editing mode and other view deleted the
slide, m_pClient was cleared in the meantime what caused
a crash

Change-Id: Ie433946799abef4c75af7b96f3e5cf9ba0e7ec47
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/113032
Tested-by: Jenkins CollaboraOffice <jenkinscollaboraoffice@gmail.com>
Reviewed-by: Jan Holesovsky <kendy@collabora.com>
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/113786
Tested-by: Jenkins
Reviewed-by: Szymon Kłos <szymon.klos@collabora.com>

sfx2/source/view/ipclient.cxx

@@ -308,7 +308,8 @@ void SAL_CALL SfxInPlaceClient_Impl::visibilityChanged( sal_Bool bVisible )
         throw uno::RuntimeException();
 
     m_pClient->GetViewShell()->OutplaceActivated( bVisible );
-    m_pClient->Invalidate();
+    if (m_pClient) // it can change in the above code
+        m_pClient->Invalidate();
 }