gcc-4.1.2-segfault: Make sure we init all ps_request_ctx_t members

When built with gcc-4.1.2, this change prevents segmentation faults
caused by undefined behaviour via uninitialized members.

Fixes https://github.com/pagespeed/ngx_pagespeed/issues/813
Overall LGTM, and we probably want to backport it once we have enough fixes for a 1.9.32.2

Any reason this doesn't have a default constructor to initialize everything?