pointing to a non-existing heap tuple, we just silently ignore it.
Such dangling index entries of course means that your database is
corrupt, but we ought to handle that better. In the worst case, the heap
slot is inserted to in the future, and then the bogus index entry points
to a wrong tuple.
ISTM we should print a warning suggesting a REINDEX, and kill the index
tuple. Killing tuples in the face of corruption is dangerous, but in
this case I think it's the right thing to do. We could also just emit
the warning, but that could fill the logs quickly if the index tuple is