FastCGI: fixed start pointers in request buffers.

The start pointers are used in ngx_http_upstream_reinit() to
reinit FastCGI requests.

src/http/modules/ngx_http_fastcgi_module.c

diff --git a/src/http/modules/ngx_http_fastcgi_module.c b/src/http/modules/ngx_http_fastcgi_module.c
index 9847ec1..a49c93c 100644
--- a/src/http/modules/ngx_http_fastcgi_module.c
+++ b/src/http/modules/ngx_http_fastcgi_module.c
@@ -1129,6 +1129,7 @@
 
                 } else {
                     b->pos = pos;
+                    b->start = pos;
                     pos += 32 * 1024;
 
                     if (pos >= body->buf->last) {