rgw: RGWCoroutine::set_sleeping() checks for null stack

rgw: RGWCoroutine::set_sleeping() checks for null stack

users of the RGWOmapAppend coroutine don't manage the lifetime of its
underlying coroutine stack, so end up making calls on RGWOmapAppend
after its stack goes away. this null check is a band-aid, and there are
still several other calls in RGWCoroutine that don't check for null
stack

Fixes: https://tracker.ceph.com/issues/49302
Signed-off-by: Or Friedmann <ofriedma@redhat.com>
Signed-off-by: Casey Bodley <cbodley@redhat.com>