aio-posix: avoid NULL pointer dereference in aio_epoll_update

aio_epoll_update dereferences parameter "node", but it could have been NULL
if deleting an fd handler that was not registered in the first place.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-id: 20161108135524.25927-2-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
