Crashes when parsing screenshots

[ximion]

Processing deb: main/c/calligra/krita-data_2.8.5+dfsg-1_all.deb
Traceback (most recent call last):
  File "/usr/local/bin/dak", line 241, in <module>
    main()
  File "/usr/local/bin/dak", line 221, in main
    module.main()
  File "/srv/dak/dak/dak/generate_metadata.py", line 1044, in main
    process_suite(suite)
  File "/srv/dak/dak/dak/generate_metadata.py", line 1002, in process_suite
    filelist, pkg_list[key])
  File "/srv/dak/dak/dak/generate_metadata.py", line 692, in read_metadata
    self.read_xml(xml_content, compdata)
  File "/srv/dak/dak/dak/generate_metadata.py", line 624, in read_xml
    screen = self.parse_screenshots_tag(subs)
  File "/srv/dak/dak/dak/generate_metadata.py", line 573, in parse_screenshots_tag
    url = usubs.text.strip()
AttributeError: 'NoneType' object has no attribute 'strip'

[ximion]

Fixed with the commit above.