fix crash when WMA is missing cover art (#75)

beets/mediafile.py

@@ -618,15 +618,15 @@ class ImageField(object):
                 return None
         
         elif obj.type == 'asf':
-            pictures = obj.mgfile['WM/Picture']
-            if pictures:
-                data = pictures[0].value
-                try:
-                    return _unpack_asf_image(data)[1]
-                except:
-                    return None
-            else:
-                return None
+            if 'WM/Picture' in obj.mgfile:
+                pictures = obj.mgfile['WM/Picture']
+                if pictures:
+                    data = pictures[0].value
+                    try:
+                        return _unpack_asf_image(data)[1]
+                    except:
+                        return None
+            return None
 
         else:
             # Here we're assuming everything but MP3, MPEG-4, and FLAC

test/test_mediafile_basic.py

@@ -198,11 +198,11 @@ READ_ONLY_CORRECT_DICTS = {
 
     'full.wma': {
         'length': 1.0,
-        'bitrate': 23,
+        'bitrate': 128000,
         'format': 'Windows Media',
         'samplerate': 44100,
         'bitdepth': 0,
-        'channels': 2,
+        'channels': 1,
     },
 }