Request body: added alert to catch duplicate body saving. If due to an error ngx_http_request_body_save_filter() is called more than once with rb->rest == 0, this used to result in a segmentation fault. Added an alert to catch such errors, just in case.

# Copyright (C) Igor Sysoev
# Copyright (C) Nginx, Inc.


cat << END >> $NGX_AUTO_CONFIG_H

#ifndef $have
#define $have  1
#endif

END