msg: Make SimpleMessenger shutdown safer

...in the case where no dispatchers were ever added, previously
we would throw an assertion in wait().

Signed-off-by: John Spray <john.spray@inktank.com>
Merge pull request #1353 from ceph/wip-messenger-shutdown

msg: Make SimpleMessenger shutdown safer

Reviewed-by: Greg Farnum <greg@inktank.com>