Reset of r->uri.len on URI parsing errors.

This ensures that debug logging and the $uri variable (if used in
400 Bad Request processing) will not try to access uninitialized
memory.

Found by Sergey Bobrov.

src/http/ngx_http_request.c

@@ -1071,6 +1071,8 @@ ngx_http_process_request_uri(ngx_http_request_t *r)
         cscf = ngx_http_get_module_srv_conf(r, ngx_http_core_module);
 
         if (ngx_http_parse_complex_uri(r, cscf->merge_slashes) != NGX_OK) {
+            r->uri.len = 0;
+
             ngx_log_error(NGX_LOG_INFO, r->connection->log, 0,
                           "client sent invalid request");
             ngx_http_finalize_request(r, NGX_HTTP_BAD_REQUEST);