sssd NULL dereference in monitor_service_shutdown

sssd may intermittently crash on shutdown due to a NULL pointer dereference in src/monitor/monitor.c:

Core was generated by `/usr/sbin/sssd -i --logger=files'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0 0x000056546b6ac93c in monitor_service_shutdown (svc=0x56546c02d280) at ../src/monitor/monitor.c:2134
2134 ctx->num_services--;

(gdb) frame
#0 0x000055647890b93c in monitor_service_shutdown (svc=0x55647a24c690) at ../src/monitor/monitor.c:2134
2134 ctx->num_services--;

(gdb) p ctx
$1 = (struct mt_ctx *) 0x0

This is Ubuntu 20.04.4:
$ lsb_release -rd
Description: Ubuntu 20.04.4 LTS
Release: 20.04

It occurs in sssd-2.2.3-3ubuntu0.8:
$ apt-cache policy sssd
sssd:
  Installed: (none)
  Candidate: 2.2.3-3ubuntu0.8
  Version table:
     2.2.3-3ubuntu0.8 500
        500 http://ports.ubuntu.com/ubuntu-ports focal-updates/main arm64 Packages
        500 http://ports.ubuntu.com/ubuntu-ports focal-security/main arm64 Packages
     2.2.3-3 500
        500 http://ports.ubuntu.com/ubuntu-ports focal/main arm64 Packages

The struct mt_ctx pointer at line 2134 of monitor.c may be set to NULL elsewhere in the code (specifically, in monitor_ctx_destructor()). Correct behavior would be to check if the pointer is NULL before dereferencing to avoid the segfault.

This still appears to be unfixed in the latest (2.2.3-3ubuntu0.9, focal-proposed) package. It was fixed upstream in sssd 2.5.2 via this commit: https://github.com/SSSD/sssd/commit/38905cac4b67f0e4c4b0f59af9ea7474482f088e

I've attached a patch that implements the upstream fix in 2.2.3-3ubuntu0.8.