The PlaylistItem ctor meant to be called from its subclass's constructor
(CollectionListItem) copied the this pointer too early. When
PlaylistItem is being constructed, by C++ rules the PlaylistItem is not
a CollectionListItem yet. This was appropriately flagged by ubsan.

I fixed this by ensuring the only user of this subclass fixes up the
pointer as soon as it can, and making it a private constructor so it's
not used elsewhere by mistake.