thermal: core: Fix thermal zone lookup by ID

When a thermal zone is looked up by an ID and no zone is found matching
that ID, the thermal_zone_get_by_id() function will return a pointer to
the thermal zone list head which isn't actually a valid thermal zone.

This can lead to a subsequent crash because a valid pointer is returned
to the called, but dereferencing that pointer as struct thermal_zone is
not safe.

Fixes: 329b064f ("thermal: core: Get thermal zone by id")
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200724170105.2705467-1-thierry.reding@gmail.com

drivers/thermal/thermal_core.c

@@ -751,16 +751,18 @@ int for_each_thermal_zone(int (*cb)(struct thermal_zone_device *, void *),
 
 struct thermal_zone_device *thermal_zone_get_by_id(int id)
 {
-	struct thermal_zone_device *tz = NULL;
+	struct thermal_zone_device *tz, *match = NULL;
 
 	mutex_lock(&thermal_list_lock);
 	list_for_each_entry(tz, &thermal_tz_list, node) {
-		if (tz->id == id)
+		if (tz->id == id) {
+			match = tz;
 			break;
+		}
 	}
 	mutex_unlock(&thermal_list_lock);
 
-	return tz;
+	return match;
 }
 
 void thermal_zone_device_unbind_exception(struct thermal_zone_device *tz,