[tor] control connection pre-auth DoS (infinite loop) with --enable-bufferevents

In control.c, this is the loop that retrieves data from the input buffer of the connection, or returns if no complete linefreed-terminated line is available (connection_fetch_from_buf_line() returns 0).

4225   while (1) {
4226     size_t last_idx;
4227     int r;
4228     /* First, fetch a line. */
4229     do {
4230       data_len = conn->incoming_cmd_len - conn->incoming_cmd_cur_len;
4231       r = connection_fetch_from_buf_line(TO_CONN(conn),
4232 …