Fix bug in parallel tidbitmap iteration.

Avoid computing idxpages[istate->spageptr] until after checking
that istate->spageptr is a legal index.

Dilip Kumar, per a report from David Rowley

Discussion: http://postgr.es/m/CAKJS1f8OtrHE+-P+=E=4ycnL29e9idZKuaTQ6o2MbhvGN9D8ig@mail.gmail.com