Attempt to fix album detail view crash

Very occasionally the album detail view fails to get the album tracks
and this causes a crash. This extra guard avoids the crash, but doesn't
fix the underlying problem.

The problem itself is very hard to track down because it's incredibly
hard to reproduce.

Persephone/Components/Browser/Album Detail/AlbumDetailView.swift

@@ -113,20 +113,21 @@ class AlbumDetailView: NSViewController {
   }
 
   func getAlbumSongs(for album: Album) {
-    App.mpdClient.getAlbumSongs(for: album.mpdAlbum) { [self] (mpdSongs: [MPDClient.MPDSong]) in
-      self.dataSource.setAlbumSongs(
-        mpdSongs.map { Song(mpdSong: $0) }
-      )
+    App.mpdClient.getAlbumSongs(for: album.mpdAlbum) { [weak self] (mpdSongs: [MPDClient.MPDSong]) in
+      guard let self = self else { return }
 
       DispatchQueue.main.async {
+        self.dataSource.setAlbumSongs(
+          mpdSongs.map { Song(mpdSong: $0) }
+        )
+
         self.albumTracksView.reloadData()
-      }
 
-      guard let song = self.dataSource.albumSongs.first?.song ??
-        self.dataSource.albumSongs[1].song
-        else { return }
+        guard !self.dataSource.albumSongs.isEmpty,
+          let song = self.dataSource.albumSongs.first?.song ??
+          self.dataSource.albumSongs[1].song
+          else { return }
 
-      DispatchQueue.main.async {
         self.getBigCoverArt(song: song, album: album)
       }
     }