[Bluetooth] Prevent crash in BLE advertising

[Bluetooth] Prevent crash in BLE advertising

bt_advertiser_h pointer was freed twice in some cases, which lead to
crashes. Setting it to a nullptr after the first destruction fixes the
problem.

[Verification] Tested crashing scenario in Chrome DevTools Console and
no crashes occurred with the change.
tct-tizen-bluetooth-tests: 100 % pass rate (auto; manual were not run,
because they don't test BluetoothLEAdapter APIs)

Change-Id: Ie0d3de744075a4c08e5266fed274e98a0d9567b8
Signed-off-by: Pawel Wasowski <p.wasowski2@samsung.com>