queue/PlaylistEdit: fix crash in SetSongIdRange() while playing

An assertion failure in UpdateQueuedSong() could trigger because the
`prev` parameter is always `nullptr`, but `queued` may be set.  And in
fact, calling UpdateQueuedSong() is only necessary when the queued
song was edited, to re-queue it with the new range.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/901

src/queue/PlaylistEdit.cxx

@@ -431,6 +431,8 @@ playlist::SetSongIdRange(PlayerControl &pc, unsigned id,
 	if (position < 0)
 		throw PlaylistError::NoSuchSong();
 
+	bool was_queued = false;
+
 	if (playing) {
 		if (position == current)
 			throw PlaylistError(PlaylistResult::DENIED,
@@ -442,6 +444,10 @@ playlist::SetSongIdRange(PlayerControl &pc, unsigned id,
 			   already; cancel that */
 			pc.LockCancel();
 			queued = -1;
+
+			/* schedule a call to UpdateQueuedSong() to
+			   re-queue the song with its new range */
+			was_queued = true;
 		}
 	}
 
@@ -464,7 +470,8 @@ playlist::SetSongIdRange(PlayerControl &pc, unsigned id,
 	song.SetEndTime(end);
 
 	/* announce the change to all interested subsystems */
-	UpdateQueuedSong(pc, nullptr);
+	if (was_queued)
+		UpdateQueuedSong(pc, nullptr);
 	queue.ModifyAtPosition(position);
 	OnModified();
 }