RTSPSouce: add check for mHandler in release

fix the media server issue in RTSP release
duplicate quit msg may cause mHandler null and it may cause crash in onDisconnect

Change-Id: Ib04158603eea3fecc70035f4054729ef8ab40dc0
Author: Yi Wang <yi.a.wang@intel.com>
Signed-off-by: Yi Wang <yi.a.wang@intel.com>
Singed-off-by: Shuo Gao <shuo.gao@intel.com>
Signed-off-by: Bruce Beare <bruce.j.beare@intel.com>
Signed-off-by: Jack Ren <jack.ren@intel.com>
Author-tracking-BZ: 49222, 44103