discard request body before returning 413 error,
this fixes custom 413 page redirection bug introduced in r1456

src/http/ngx_http_core_module.c

diff --git a/src/http/ngx_http_core_module.c b/src/http/ngx_http_core_module.c
index 8d83c44..f99b910 100644
--- a/src/http/ngx_http_core_module.c
+++ b/src/http/ngx_http_core_module.c
@@ -917,6 +917,7 @@
                       "client intended to send too large body: %O bytes",
                       r->headers_in.content_length_n);
 
+        (void) ngx_http_discard_request_body(r);
         ngx_http_finalize_request(r, NGX_HTTP_REQUEST_ENTITY_TOO_LARGE);
         return NGX_OK;
     }