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.

--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -2287,6 +2287,7 @@ ngx_http_upstream_send_response(ngx_http
             return;
         }
 
+        p->buf_to_file->start = u->buffer.start;
         p->buf_to_file->pos = u->buffer.start;
         p->buf_to_file->last = u->buffer.pos;
         p->buf_to_file->temporary = 1;