presenter console: fix STL assert when exiting slide show

AtkListener::handleChildRemoved() calls removeAccessibleEventListener(),
apparently without the listener being registered, so the remove() returns
end(), which isn't valid for erase().

(probably happens since e4e208fa2b)

Change-Id: Ia0de1bafe214f5a46f250d6573d8c4b408c9cf26

sdext/source/presenter/PresenterAccessibility.cxx

@@ -1109,7 +1109,11 @@ void SAL_CALL PresenterAccessible::AccessibleObject::removeAccessibleEventListen
     {
         const osl::MutexGuard aGuard(m_aMutex);
 
-        maListeners.erase(std::remove(maListeners.begin(), maListeners.end(), rxListener));
+        auto const it(std::remove(maListeners.begin(), maListeners.end(), rxListener));
+        if (it != maListeners.end())
+        {
+            maListeners.erase(it);
+        }
     }
 }