Description of problem:
Issue can be reproduced with the following foo@.service unit file:

    [Unit]
    
    [Service]
    Type=oneshot
    
    UtmpIdentifier=%I
    UtmpMode=user
    
    ExecStart=/usr/bin/sleep 30

My motivation was writing a unit for the Cage compositor based on the docs:
  https://github.com/cage-kiosk/cage/wiki/Starting-Cage-on-boot-with-systemd

I realised (after causing the crash) that my error was that I'd omitted
the User= config option.  Regardless, that shouldn't cause a crash.


Version-Release number of selected component:
systemd-254.9-1.fc39

Additional info:
reporter:       libreport-2.17.11
type:           CCpp
reason:         systemd killed by SIGSEGV
journald_cursor: s=6c97792b866141b8a1e857811ba3e54b;i=e31e;b=583195a7f1b64415b2590e3238f77a22;m=f90c632a;t=61168fb4c614e;x=47952259703ac99f
executable:     /usr/lib/systemd/systemd
cmdline:        $'(cage)'
cgroup:         0::/system.slice/system-cage\x2dvm.slice/cage-vm
rootdir:        /
uid:            0
kernel:         6.7.4-200.fc39.x86_64
package:        systemd-254.9-1.fc39
runlevel:       N 5
backtrace_rating: 3
crash_function: __strncpy_sse2_unaligned

Truncated backtrace:
Thread no. 1 (17 frames)
 #0 __strncpy_sse2_unaligned
 #1 strncpy at /usr/include/bits/string_fortified.h:95
 #2 utmp_put_init_process at ../src/shared/utmp-wtmp.c:209
 #3 exec_child at ../src/core/execute.c:5236
 #4 exec_spawn at ../src/core/execute.c:6075
 #5 service_spawn_internal at ../src/core/service.c:1816
 #6 service_enter_start at ../src/core/service.c:2393
 #7 service_enter_condition at ../src/core/service.c:2497
 #8 service_start at ../src/core/service.c:2754
 #9 job_perform_on_unit at ../src/core/job.c:854
 #10 job_run_and_invalidate at ../src/core/job.c:927
 #11 manager_dispatch_run_queue at ../src/core/manager.c:2386
 #13 source_dispatch at ../src/libsystemd/sd-event/sd-event.c:4227
 #14 sd_event_dispatch at ../src/libsystemd/sd-event/sd-event.c:4808
 #15 sd_event_run at ../src/libsystemd/sd-event/sd-event.c:4869
 #16 manager_loop at ../src/core/manager.c:3224
 #17 invoke_main_loop at ../src/core/main.c:2012