Don't exaggerate the number of temporary blocks read.

A read that returns zero bytes (or an error) should not increment
the number of temporary blocks read.

Thomas Munro

Discussion: http://postgr.es/m/CAEepm=21xgihg=WaG+O5MFotEZfN6kFETpfw+RkSnEqNQqGn2Q@mail.gmail.com