io/channel-websock: Replace strlen(const_str) by sizeof(const_str) - 1

io/channel-websock: Replace strlen(const_str) by sizeof(const_str) - 1

The combined_key[... QIO_CHANNEL_WEBSOCK_GUID_LEN ...] array in
qio_channel_websock_handshake_send_res_ok() expands to a call
to strlen(QIO_CHANNEL_WEBSOCK_GUID), and the compiler doesn't
realize the string is const, so consider combined_key[] being
a variable-length array.

To remove the variable-length array, we provide it a hint to
the compiler by using sizeof() - 1 instead of strlen().

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20220819153931.3147384-5-peter.maydell@linaro.org