Bug#856985: systemd: Uninstalling systemd-cron hangs/crashes systemd

[Andreas Bombe]

It was pointed out to me that there should be core dumps and indeed
there were from both instances of it happening to me. Here is one
backtrace, the other looks identical:

#0  0x00007fc21779775b in raise (sig=6) at ../nptl/sysdeps/unix/sysv/linux/pt-raise.c:37
#1  0x00007fc217bed4a8 in crash.lto_priv.235 (sig=6) at ../src/core/main.c:158
#2  <signal handler called>
#3  0x00007fc217412067 in __GI_raise (sig=sig at entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
#4  0x00007fc217413448 in __GI_abort () at abort.c:89
#5  0x00007fc217c95ec2 in log_assert_failed (text=text at entry=0x7fc217cbef00 "s->exec_command[SERVICE_EXEC_START]", 
    file=file at entry=0x7fc217cb8202 "../src/core/service.c", line=line at entry=1312, 
    func=func at entry=0x7fc217cbf8c0 <__PRETTY_FUNCTION__.15381> "service_enter_start") at ../src/shared/log.c:709
#6  0x00007fc217c65c0f in service_enter_start (s=s at entry=0x7fc218349dc0) at ../src/core/service.c:1312
#7  0x00007fc217c67d41 in service_sigchld_event.lto_priv.376 (u=0x7fc218349dc0, pid=<optimized out>, code=<optimized out>, status=0)
    at ../src/core/service.c:2337
#8  0x00007fc217c9ab87 in manager_dispatch_sigchld (m=0x7fc2182d9380) at ../src/core/manager.c:1639
#9  0x00007fc217c9c645 in manager_dispatch_signal_fd.lto_priv.926 (source=0x1, fd=1, revents=6, userdata=0x7fc2182d9380)
    at ../src/core/manager.c:1890
#10 0x00007fc217bf10b0 in source_dispatch (s=0x7fc2182d9a50) at ../src/libsystemd/sd-event/sd-event.c:2016
#11 0x00007fc217bf22ff in sd_event_run (e=0x7fc2182d81a0, timeout=<optimized out>) at ../src/libsystemd/sd-event/sd-event.c:2314
#12 0x00007fc217c9b951 in manager_loop (m=0x7fc2182d9380) at ../src/core/manager.c:2009
#13 0x00007fc217be9a56 in main (argc=3, argv=<optimized out>) at ../src/core/main.c:1748