starmath: dispose SmEditController object correctly

(cherry-picked from 17cdc92e)

Change-Id: I5d44ff6d514d2e40df23f6c419463b980f39d2c7
Reviewed-on: https://gerrit.libreoffice.org/16287Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

starmath/source/view.cxx

@@ -727,6 +727,7 @@ void SmCmdBoxWindow::dispose()
 {
     aInitialFocusTimer.Stop();
     bExiting = true;
+    aController.dispose();
     aEdit.disposeAndClear();
     SfxDockingWindow::dispose();
 }