Request body: block reading when returning errors. If reading is not blocked, additional client activity can result in undefined behaviour, including segfaults, as seen with proxying with proxy_ignore_client_abort before 4072:cf334deeea66. While unlikely with low-level errors being returned when reading the request body, it might be the case, for example, when chunked transfer encoding is used, and error_page 400 (or 413) is proxied with proxy_ignore_client_abort.