Fix playlist edit after edit race

After an edit option the playlist is thawed and triggers
the butler thread (SessionEvent::Overwrite) to re-read the
playlist.

If another edit operation is started the butler may reads a
region using the region's new position, but the playlist's
old range.

See also 4db1c02bd1

libs/ardour/playlist.cc

@@ -449,6 +449,10 @@ Playlist::end_undo ()
 void
 Playlist::freeze ()
 {
+	/* flush any ongoing reads, paricularly AudioPlaylist::read(),
+	 * before beginning to modify the playlist.
+	 */
+	RegionWriteLock rlock (this);
 	delay_notifications ();
 	g_atomic_int_inc (&ignore_state_changes);
 }