cpufreq: CPPC: Fix possible null-ptr-deref for cppc_get_cpu_cost()

cpufreq_cpu_get_raw() may return NULL if the cpu is not in
policy->cpus cpu mask and it will cause null pointer dereference,
so check NULL for cppc_get_cpu_cost().

Fixes: 740fcdc ("cpufreq: CPPC: Register EM based on efficiency class information")
Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>

Author: Jinjie Ruan

drivers/cpufreq/cppc_cpufreq.c

@@ -474,6 +474,9 @@ static int cppc_get_cpu_cost(struct device *cpu_dev, unsigned long KHz,
 	int step;
 
 	policy = cpufreq_cpu_get_raw(cpu_dev->id);
+	if (!policy)
+		return 0;
+
 	cpu_data = policy->driver_data;
 	perf_caps = &cpu_data->perf_caps;
 	max_cap = arch_scale_cpu_capacity(cpu_dev->id);