udevd: don't use monitor after manager_exit()

udevd: don't use monitor after manager_exit()

If udevd receives an exit signal, it releases its reference on the udev
monitor in manager_exit(). If at this time a worker is hanging, and if
the event timeout for this worker expires before udevd exits, udevd
crashes in on_sigchld()->udev_monitor_send_device(), because the monitor
has already been freed.

Fix this by testing the validity of manager->monitor in on_sigchld().