fix search/DB server disagreement crash (GC-418)

beets/autotag/mb.py

@@ -242,8 +242,8 @@ def match_album(artist, album, tracks=None, limit=SEARCH_LIMIT):
         # The search result is missing some data (namely, the tracks),
         # so we just use the ID and fetch the rest of the information.
         albuminfo = album_for_id(release['id'])
-        assert albuminfo is not None
-        yield albuminfo
+        if albuminfo is not None:
+            yield albuminfo
 
 def match_track(artist, title, limit=SEARCH_LIMIT):
     """Searches for a single track and returns an iterable of TrackInfo

docs/changelog.rst

@@ -7,6 +7,8 @@ Changelog
 * :doc:`/plugins/fetchart`: Fix a bug where cover art filenames could lack
   a ``.jpg`` extension.
 * Add the track mapping dictionary to the ``album_distance`` plugin function.
+* Fix an assertion failure when the MusicBrainz main database and search server
+  disagree.
 
 1.0b15 (July 26, 2012)
 ----------------------