irqchip/mbigen: Fix potential NULL dereferencing

irqchip/mbigen: Fix potential NULL dereferencing

platform_get_resource() may return NULL, add proper
check to avoid potential NULL dereferencing.

Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: linuxarm@huawei.com
Cc: Wei Yongjun <weiyongjun1@huawei.com>
Cc: MaJun <majun258@huawei.com>
Link: http://lkml.kernel.org/r/1494561328-39514-3-git-send-email-guohanjun@huawei.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>