core: update invoke_sigchld_event() to handle NULL ->sigchld_event()

After receiving SIGCHLD, one of the ways manager_dispatch_sigchld() maps the
now zombie $PID to its unit is through manager_get_unit_by_pid_cgroup() which
reads /proc/$PID/cgroup and looks up the unit associated with the cgroup path.

On non-unified cgroup hierarchies, a process is immediately migrated to the
root cgroup on death and the cgroup lookup would always have returned the unit
associated with it, making it rather pointless but safe.  On unified hierarchy,
a zombie remains associated with the cgroup that it was associated with at the
time of death and thus manager_get_unit_by_pid_cgroup() will look up the unit
properly.

However, by the time manager_dispatch_sigchld() is running, the original cgroup
may have become empty and it and its associated unit might already have been
removed.  If the cgroup path doesn't yield a match, manager_dispatch_sigchld()
keeps pruning the leaf component.  This means that the function may return a
slice unit for a pid and as a slice doesn't have ->sigchld_event() handler,
calling invoke_sigchld_event() on it causes a segfault.

This patch updates invoke_sigchld_event() so that it skips calling if the
handler is not set.

src/core/manager.c

@@ -1631,7 +1631,9 @@ static void invoke_sigchld_event(Manager *m, Unit *u, const siginfo_t *si) {
         log_unit_debug(u, "Child "PID_FMT" belongs to %s", si->si_pid, u->id);
 
         unit_unwatch_pid(u, si->si_pid);
-        UNIT_VTABLE(u)->sigchld_event(u, si->si_pid, si->si_code, si->si_status);
+
+        if (UNIT_VTABLE(u)->sigchld_event)
+                UNIT_VTABLE(u)->sigchld_event(u, si->si_pid, si->si_code, si->si_status);
 }
 
 static int manager_dispatch_sigchld(Manager *m) {