Preemptively release pendingMapObjects upon Map destruction

In this way QGeoMap::mapObjects doesn't have to return all map objects,
as the user added ones should be covered by m_pendingMapObjects.

Change-Id: I5c850f2fb820629e0c4dd941d13f04cb3684b898
Reviewed-by: BogDan Vatra <bogdan@kdab.com>