cpufreq: stats: time_in_state can't be NULL in cpufreq_stats_update()

'time_in_state' can't be NULL if 'stats' is valid. These are allocated together
and only if time_in_state is allocated successfully, we update policy->stats.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>