[SSSD] [sssd PR#5599][synchronized] monitor: avoid NULL deref in monitor_service_shutdown()

[alexey-tikhonov]

   URL: https://github.com/SSSD/sssd/pull/5599
Author: alexey-tikhonov
 Title: #5599: monitor: avoid NULL deref in monitor_service_shutdown()
Action: synchronized

To pull the PR as Git branch:
git remote add ghsssd https://github.com/SSSD/sssd
git fetch ghsssd pull/5599/head:pr5599
git checkout pr5599