gatt: Fix possible crashes when disconnecting

gatt: Fix possible crashes when disconnecting

If there are pending AcquireWrite or AcquireNotify when disconnecting
the attribute object may be freed (e.g. device is temporary) leading to
the following backtrace:

bluetoothd[369928]: src/gatt-database.c:gatt_db_service_removed() Local GATT service removed
bluetoothd[369928]: src/adapter.c:adapter_service_remove() /org/bluez/hci1
bluetoothd[369928]: src/adapter.c:remove_uuid() sending remove uuid command for index 1
bluetoothd[369928]: src/sdpd-service.c:remove_record_from_server() Removing record with handle 0x1002e
bluetoothd[369928]: src/gatt-database.c:send_notification_to_device() GATT server sending indication
bluetoothd[369928]: src/device.c:gatt_debug() Write Complete: err -125
bluetoothd[369928]: src/gatt-database.c:client_disconnect_cb() Client disconnected
bluetoothd[369928]: src/advertising.c:client_disconnect_cb() Client disconnected
bluetoothd[369928]: Failed to acquire write: org.freedesktop.DBus.Error.NoReply

Program received signal SIGSEGV, Segmentation fault.
0x0000555555631450 in acquire_write_reply (message=0x55555583dec0, user_data=0x555555843e40) at src/gatt-database.c:2437
2437            send_write(op->device, op->attrib, chrc->proxy, NULL, op->id,

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