core/advertising: Fix crash when unregistering advertisement too fast

core/advertising: Fix crash when unregistering advertisement too fast

When advertisement is unregistered during MGMT_OP_ADD_ADVERTISING it will
crash in add_adv_callback because struct btd_adv_client no longer exist.

This is seen also in debug log from bluetoothd:
bluetoothd[29698]: src/advertising.c:register_advertisement() RegisterAdvertisement
bluetoothd[29698]: src/advertising.c:client_create() Adding proxy for /org/bluez/example/advertisement0
bluetoothd[29698]: src/advertising.c:register_advertisement() Registered advertisement at path /org/bluez/example/advertisement0
bluetoothd[29698]: src/advertising.c:parse_service_uuids() Adding ServiceUUID: 180D
bluetoothd[29698]: src/advertising.c:parse_service_uuids() Adding ServiceUUID: 180F
bluetoothd[29698]: src/advertising.c:parse_manufacturer_data() Adding ManufacturerData for ffff
bluetoothd[29698]: src/advertising.c:parse_service_data() Adding ServiceData for 9999
bluetoothd[29698]: src/advertising.c:parse_data() Adding Data for type 0x26 len 3
bluetoothd[29698]: src/advertising.c:refresh_adv() Refreshing advertisement: /org/bluez/example/advertisement0
bluetoothd[29698]: src/advertising.c:unregister_advertisement() UnregisterAdvertisement
bluetoothd[29698]: src/advertising.c:add_adv_callback() Advertisement registered: �
Segmentation fault (core dumped)

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