cpufreq: stats: Don't update cpufreq_stats_table if it's NULL

When cpufreq_stats_table is not initialized for policy->last_cpu in
CPUFREQ_UPDATE_POLICY_CPU callback, no updates are necessary. Current
implementation dereferences the table unconditionally, causing a crash
if the table is NULL.

Return directly in cpufreq_stats_update_policy_cpu() if
cpufreq_stats_table of policy->last_cpu is NULL.

Change-Id: Ic9ef8120557702791ba5b873532e47cc0a5dc027
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>