h2: use after free on premature connection close #920

lib/http2/connection.c:on_read() calls parse_input(), which might free
`conn`. It does so in particular if the connection preface isn't
the expected one in expect_preface(). `conn` is then used after the free
in `if (h2o_timeout_is_linked(&conn->_write.timeout_entry)`.
We fix this by adding a return value to close_connection that returns a
negative value if `conn` has been free'd and can't be used anymore.

Credits for finding the bug to Tim Newsham.
static void enqueue_goaway(h2o_http2_conn_t *conn, int errnum, h2o_iovec_t additional_data)
{
    if (conn->state < H2O_HTTP2_CONN_STATE_IS_CLOSING) {
        /* http2 spec allows sending GOAWAY more than once (for one reason since errors may arise after sending the first one) */
        h2o_http2_encode_goaway_frame(&conn->_write.buf, conn->pull_stream_ids.max_open, errnum, additional_data);
        h2o_http2_conn_request_write(conn);
        conn->state = H2O_HTTP2_CONN_STATE_HALF_CLOSED;
    }
}

static void enqueue_goaway(h2o_http2_conn_t *conn, int errnum, h2o_iovec_t additional_data)
{
    if (conn->state < H2O_HTTP2_CONN_STATE_IS_CLOSING) {
        /* http2 spec allows sending GOAWAY more than once (for one reason since errors may arise after sending the first one) */
        h2o_http2_encode_goaway_frame(&conn->_write.buf, conn->pull_stream_ids.max_open, errnum, additional_data);
        h2o_http2_conn_request_write(conn);
        conn->state = H2O_HTTP2_CONN_STATE_HALF_CLOSED;
    }
}
