adapter: Fix possible crash when stopping discovery

adapter: Fix possible crash when stopping discovery

If the client disconnect/crash while MGMT_OP_STOP_DISCOVERY was pending
it would possibly cause a crash as the client pointer is passed to
mgmt_send and accessed in the callback after being freed.

To fix this the adapter itself is now passed to the callback so the
client is not accessed directly, instead the code now checks if
discovery_list has not been cleared in the meantime and only then
proceed to access the client pointer which is how
MGMT_OP_START_DISCOVERY is handled.

Signed-off-by: Anuj Jain <anuj01.jain@samsung.com>
Signed-off-by: Ayush Garg <ayush.garg@samsung.com>