bthost_ipc: fix potential NPE in audio_check_a2dp_ready

if audio_check_a2dp_ready is retrying to check a2dp ready,
and bt_stack_deinit is called, stack_cb will be null and
trigger NPE.

Change-Id: Ib3192ac3c958ac71b77e6a6f1c48c4d39f3d2dc4
CRs-Fixed: 2264111
1 file changed
tree: f592fba7bd94e90ac63cc9a1bc8f6b3afd357645
  1. bthost_ipc/
  2. libbt-vendor/
  3. tools/
  4. Android.mk
  5. CleanSpec.mk