cpufreq/longhaul: handle NULL policy in longhaul_exit

In the Linux kernel, the following vulnerability has been resolved: cpufreq/longhaul: handle NULL policy in longhaul_exit longhaul_exit() was calling cpufreq_cpu_get(0) without checking for a NULL policy pointer. On some systems, this could lead to a NULL dereference and a kernel warning or panic. This patch adds a check using unlikely() and returns early if the policy is NULL. Bugzilla: #219962