Primarily, this didn't work correctly when the tidbitmap ended up
empty.

Dilip Kumar, per a report from Emre Hasegeli

Discussion: http://postgr.es/m/CAFiTN-ujHFKb8WSLhK54rfqQT3r2yiPQOyeBrCDsA4p9Fwp_jw@mail.gmail.com