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 on_idle_timeout(h2o_timeout_entry_t *entry)
{
    h2o_http2_conn_t *conn = H2O_STRUCT_FROM_MEMBER(h2o_http2_conn_t, _timeout_entry, entry);

    enqueue_goaway(conn, H2O_HTTP2_ERROR_NONE, h2o_iovec_init(H2O_STRLIT("idle timeout")));
    close_connection(conn);
}

static void on_idle_timeout(h2o_timeout_entry_t *entry)
{
    h2o_http2_conn_t *conn = H2O_STRUCT_FROM_MEMBER(h2o_http2_conn_t, _timeout_entry, entry);

    enqueue_goaway(conn, H2O_HTTP2_ERROR_NONE, h2o_iovec_init(H2O_STRLIT("idle timeout")));
    close_connection(conn);
}
