Event pipe: fixed handling of buf_to_file data. Input filter might free a buffer if there is no data in it, and in case of first buffer (used for cache header and request header, aka p->buf_to_file) this resulted in cache corruption. Buffer memory was reused to read upstream response before headers were written to disk. Fix is to avoid moving pointers in ngx_event_pipe_add_free_buf() to a buffer start if we were asked to free a buffer used by p->buf_to_file. This fixes occasional cache file corruption, usually resulted in "cache file ... has md5 collision" alerts. Reported by Anatoli Marinov.