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

In the Linux kernel, the following vulnerability has been resolved: 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().