Do not add cover art relation url twice (ie. cdbaby)

Stop loop if first url is added because second one will be removed later anyway.
It happens with cdbaby urls (ie. http://musicbrainz.org/release/6e228dfa-b0c7-4987-a36d-7ac14541ae66)


Debug output without this patch:
D: 139947473626880 11:15:32 There are no suitable images in the cover art archive for 6e228dfa-b0c7-4987-a36d-7ac14541ae66
D: 139947473626880 11:15:32 Adding front image PyQt4.QtCore.QUrl(u'http://cdbaby.name/p/r/prozak.jpg')
D: 139947473626880 11:15:32 Adding front image PyQt4.QtCore.QUrl(u'http://www.cdbaby.com/cd/prozak')
D: 139947473626880 11:15:32 'Downloading http://cdbaby.name:80/p/r/prozak.jpg'


Debug output with this patch:
D: 140074644940544 11:16:55 There are no suitable images in the cover art archive for 6e228dfa-b0c7-4987-a36d-7ac14541ae66
D: 140074644940544 11:16:55 Adding front image PyQt4.QtCore.QUrl(u'http://cdbaby.name/p/r/prozak.jpg')
D: 140074644940544 11:16:55 'Downloading http://cdbaby.name:80/p/r/prozak.jpg'