cpuidle: Avoid NULL argument in cpuidle_switch_governor()

Checks if the new governor is NULL before updating the
cupidle_curr_governor.

Signed-off-by: gaurav jindal <gauravjindal1104@gmail.com>
[ rjw : Subject ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>