vpc: prevent runtime exception

(cherry picked from commit b31c2a76e6116022165f0f2c1e2ea7b38c8713b7)

Sources/VLCPlaybackController+MediaLibrary.m

@@ -54,7 +54,8 @@
     NSMutableArray *files = [NSMutableArray arrayWithCapacity:tracks.count];
     for (MLAlbumTrack *track in tracks) {
         MLFile *file = track.files.anyObject;
-        [files addObject:file];
+        if (file)
+            [files addObject:file];
     }
     int index = (int)[tracks indexOfObject:albumTrack];
     [self configureMediaListWithFiles:files indexToPlay:index];
@@ -76,5 +77,4 @@
     self.pathToExternalSubtitlesFile = nil;
 }
 
-
 @end