rgw: clear realm watch on failed watch_restart

rgw: clear realm watch on failed watch_restart

if the realm is deleted while the gateway has a watch, it's disconnected
and watch_restart() will fail. this results in a watch handle of 0,
which leads to a segfault on ~RGWRealmWatcher when we pass it to
unwatch()

this commit cleans up the watch when watch_restart() fails, so we don't
try to unwatch() on destruction

Fixes: http://tracker.ceph.com/issues/16817
Signed-off-by: Casey Bodley <cbodley@redhat.com>