Fixing segfault if filtered_cloud_topic is not set in PointCloudOctomapUpdater

Iterators were created into a PointCloud2 message which was only created if filtered_cloud_topic was set. Furthermore, the incoming data was never used during the update - the octomap update iterated over the output PointCloud2, as opposed to the input PointCloud 2. On my setup (Trusty/Indigo), PointCloudOctomapUpdater would crash immediately unless I defined filtered_cloud_topic, and I would never see any output data. I believe this pull request fixes this issue.