libstagefright: check the RTCP socket valid before sending RR

make sure the rtcp socket is valid before sending RR to avoid assertion failure

Change-Id: Ied376107e2397bf5888af42ff76c7f82d465a216
Author: Yi Wang <yi.a.wang@intel.com>
Signed-off-by: Yi Wang <yi.a.wang@intel.com>
Signed-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: 49305, 18533, 16913