kded/devicemonitor: Check BlueDevilDaemon exists when calling login1PrepareForSleep
It's possible that BlueDevilDaemon has been destroyed, but PrepareForSleep is received before DeviceMonitor is destroyed, so a crash will happen. To prevent the crash, add a check in login1PrepareForSleep to validate BlueDevilDaemon still exists.
Edited by Fushan Wen