Crash if Evolution address book is not available

[Luca Falavigna]

Kupfer generates a traceback if Evolution address book isn't available:

** (kupfer.py:11129): WARNING **: Failed to alloc new addressbook: Cannot open book
Exception in thread Thread-13:
Traceback (most recent call last):
  File "/usr/lib/python2.6/threading.py", line 532, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.6/threading.py", line 484, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/usr/share/kupfer/kupfer/core/sources.py", line 75, in rescan_source
    list(source.get_leaves(force_update=force_update))
  File "/usr/share/kupfer/kupfer/obj/grouping.py", line 76, in get_leaves
    leaves = Source.get_leaves(src, force_update)
  File "/usr/share/kupfer/kupfer/obj/base.py", line 409, in get_leaves
    self.cached_items = aslist(sort_func(self.get_items_forced()))
  File "/usr/share/kupfer/kupfer/utils.py", line 70, in locale_sort
    seq = seq if isinstance(seq, list) else list(seq)
  File "/usr/share/kupfer/kupfer/plugin/evolution.py", line 107, in get_items
    for contact in ebook_.get_all_contacts():
AttributeError: 'NoneType' object has no attribute 'get_all_contacts

This was also reported as http://bugs.debian.org/606956

Attached patch is a workaround for this issue.

[X (u78qir8a9-deactivatedaccount)]

Thanks a lot