Open bug in launchpad.net:
https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1341623

Crash.

#0  0x00007f179fb4c117 in __GI_raise (sig=sig@entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
        resultvar = 0
        pid = 1045
        selftid = 1045
#1  0x00007f179fb4d808 in __GI_abort () at abort.c:89
        save_stage = 2
        act = {__sigaction_handler = {sa_handler = 0x8a64d0, sa_sigaction = 0x8a64d0}, sa_mask = {__val = {9137825, 4354759, 9137552, 4350346, 1, 4294967296, 4322088, 9139344, 206158430256, 140737480192464, 140737480192256, 0, 139739453006202, 0, 140737480192368, 9137520}}, sa_flags = 9137825, sa_restorer = 0x4272c7}
        sigs = {__val = {32, 0 <repeats 15 times>}}
#2  0x00007f17a0537309 in nih_free (ptr=0x0) at alloc.c:318
        ctx = <optimized out>
        __FUNCTION__ = "nih_free"
#3  0x0000000000422ae0 in cgm_get (controller=0x4272c7 "systemd", cgroup_path=0x8b6ea1 "user/1000.user/c2.session", cgroup_path@entry=0x8b6ea0 "/user/1000.user/c2.session", key=key@entry=0x433f05 "tasks") at ../src/shared/cgmanager.c:143
        nerr = 0x8b6d90
        result = 0x0
        ret = 0x0
        __func__ = "cgm_get"
#4  0x0000000000420aa7 in cg_enumerate_tasks (controller=0x42618a "name=systemd", path=0x8b6ea0 "/user/1000.user/c2.session", _f=_f@entry=0x7fffff837248) at ../src/shared/cgroup-util.c:128
        fs = 0x0
        f = <optimized out>
        r = <optimized out>
        value = 0x0
        template = 0x0
        fd = <optimized out>
        __PRETTY_FUNCTION__ = "cg_enumerate_tasks"
#5  0x0000000000420c6a in cg_is_empty (controller=<optimized out>, path=<optimized out>, ignore_self=true) at ../src/shared/cgroup-util.c:1125
        f = 0x0
        pid = 0
        self_pid = <optimized out>
        found = false
        r = <optimized out>
#6  0x0000000000420dcb in cg_is_empty_recursive (controller=controller@entry=0x42618a "name=systemd", path=0x8b6ea0 "/user/1000.user/c2.session", ignore_self=ignore_self@entry=true) at ../src/shared/cgroup-util.c:1171
        d = 0x0
        fn = 0x18 <error: Cannot access memory at address 0x18>
        r = <optimized out>
        children = 0x0
        p1 = <optimized out>
        i = 0
        __PRETTY_FUNCTION__ = "cg_is_empty_recursive"
#7  0x000000000040feab in session_terminate_cgroup (s=0x8b6960) at ../src/login/logind-session.c:651
        r = <optimized out>
        k = <optimized out>
#8  session_stop (s=s@entry=0x8b6960) at ../src/login/logind-session.c:709
        __PRETTY_FUNCTION__ = "session_stop"
        __func__ = "session_stop"
#9  0x0000000000409630 in manager_dispatch_other (fd=23, m=0x8a5010) at ../src/login/logind.c:1262
        s = 0x8b6960
        i = <optimized out>
        b = <optimized out>
#10 manager_run (m=m@entry=0x8a5010) at ../src/login/logind.c:1795
        event = {events = 16, data = {ptr = 0x1d, fd = 29, u32 = 29, u64 = 29}}
        n = <optimized out>
        msec = <optimized out>
        __PRETTY_FUNCTION__ = "manager_run"
        __func__ = "manager_run"
#11 0x000000000040622c in main (argc=<optimized out>, argv=<optimized out>) at ../src/login/logind.c:1872
        m = 0x8a5010
        r = 0
        __func__ = "main"