When passing query strings to workers, pass the terminating \0.

When passing query strings to workers, pass the terminating \0.

Otherwise, when the query string is read, we might trailing garbage
beyond the end, unless there happens to be a \0 there by good luck.

Report and patch by Thomas Munro. Reviewed by Rafia Sabih.

Discussion: http://postgr.es/m/CAEepm=2SJs7X+_vx8QoDu8d1SMEOxtLhxxLNzZun_BvNkuNhrw@mail.gmail.com