Fix igraph issue
    
    The problem was, that the way we originaly created the graph (from an
    edgelist) caused errors of "unknown vertices" when looking for some
    paths.
    This happened, when the orphans.json file's `affected` section had
    something like this at the end:
    ```
            "usb_modeswitch-data": [
                "usb_modeswitch"
            ],
            "viewmtn": [],
            "webacula": [],
            "wicd-kde": []
    ```
    The old code failed with exception when trying to find "shortest path"
    to the `viewmtn` node.
    
    The root of the problem was the way ve created the graph - from a list
    of edges, where an edge is  `"usb_modeswitch-data" -> "usb_modeswitch"`
    but not `"viewmtn" -> None`. The vertices are internally represented as
    integers, and since the "largest" value was the one corresponding to the
    `"usb_modeswitch-data"` vertex, the `"viewmtn"` was "out of bounds".
    
    The fix simply makes sure to tell the iGraph the number of all vertices,
    so the last ones are "covered".