Fixed buffer over-read while logging invalid request headers.

Fixed buffer over-read while logging invalid request headers.

Since 667aaf61a778 (1.1.17) the ngx_http_parse_header_line() function can return
NGX_HTTP_PARSE_INVALID_HEADER when a header contains NUL character.  In this
case the r->header_end pointer isn't properly initialized, but the log message
in ngx_http_process_request_headers() hasn't been adjusted.  It used the pointer
in size calculation, which might result in up to 2k buffer over-read.

Found with afl-fuzz.