drivers: thermal: add check when unregistering cpu cooling

This patch avoids NULL pointer accesses while unregistering
cpu cooling devices, in case a NULL pointer is received.

Cc: Zhang Rui <rui.zhang@intel.com>
Cc: linux-pm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: NEduardo Valentin <eduardo.valentin@ti.com>

drivers/thermal/cpu_cooling.c

@@ -498,8 +498,12 @@ EXPORT_SYMBOL_GPL(cpufreq_cooling_register);
  */
 void cpufreq_cooling_unregister(struct thermal_cooling_device *cdev)
 {
-	struct cpufreq_cooling_device *cpufreq_dev = cdev->devdata;
+	struct cpufreq_cooling_device *cpufreq_dev;
 
+	if (!cdev)
+		return;
+
+	cpufreq_dev = cdev->devdata;
 	mutex_lock(&cooling_cpufreq_lock);
 	cpufreq_dev_count--;