cpufreq: scpi: Fix null-ptr-deref in scpi_cpufreq_get_rate()

In the Linux kernel, the following vulnerability has been resolved: cpufreq: scpi: Fix null-ptr-deref in scpi_cpufreq_get_rate() cpufreq_cpu_get_raw() can return NULL when the target CPU is not present in the policy->cpus mask. scpi_cpufreq_get_rate() does not check for this case, which results in a NULL pointer dereference.