When ipi mailbox is used, cpu_index is decoded from iocsr register.
cpu maybe does not exist. This patch adds NULL pointer check on
ipi device.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Song Gao <gaosong@loongson.cn>
Message-Id: <20230512100421.1867848-4-gaosong@loongson.cn>