Don't show availability bar for magnet links

This commit fixes an exception in PropertiesWidget:

"Caught exception in PropertiesWidget::loadDynamicData():  invalid
torrent handle used"

This exception occurs when an user clicks on a torrent that doesn't have
a metadata (when a magnet link is not resolved yet).

One should not call torrent_handle::get_torrent_info when torrent
doesn't have a metadata.