Add null pointer check in A2DPService

A call to the stop() function in A2DPService.java
could cause a NullPointerException in certain events.
This patch adds a null pointer check before a
function call to avoid this issue.

Change-Id: I39b521592d9cc5b910c5ce232d0d8d03389a0391
Orig-Change-Id: I77ee8129c3d81230afc5b59b4cc29e690cbe547a
Author: Thomas Lehaux <thomasx.lehaux@intel.com>
Reviewed-on: http://android.intel.com:8080/102211
Reviewed-by: Poignet, Cedric <cedric.poignet@intel.com>
Reviewed-by: cactus <cactus@intel.com>
Reviewed-by: Bondier, CedricX <cedricx.bondier@intel.com>
Reviewed-by: Boniface, XavierX <xavierx.boniface@intel.com>
Tested-by: Zurmely, PierreX <pierrex.zurmely@intel.com>
Reviewed-by: buildbot <buildbot@intel.com>
Tested-by: buildbot <buildbot@intel.com>
Signed-off-by: Jian Luo <jian.luo@intel.com>
Signed-off-by: Bruce Beare <bruce.j.beare@intel.com>
Signed-off-by: Jack Ren <jack.ren@intel.com>
Author-tracking-BZ: 85837