Don't crash in _checkImageState if there's no image.os_plugin
    
    We've seen some tasks lately where Koji crashed trying to do
    `image.os_plugin.abort()`, e.g.:
    
    https://koji.fedoraproject.org/koji/taskinfo?taskID=89750587
    https://koji.fedoraproject.org/koji/taskinfo?taskID=89671944
    
    The fix seems fairly simple: only try and abort if os_plugin
    actually exists.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>