[Powertop] [PATCH] Fix backlight usage reporting when backlight control is not supported

On PC file /sys/class/backlight/.../max_brightness can contain 0
when backlight control is not supported. Added check helps to avoid
dividing by zero and printing NaN.
---
 src/devices/backlight.cpp |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/src/devices/backlight.cpp b/src/devices/backlight.cpp
index b8c9147..82cb1e0 100644
--- a/src/devices/backlight.cpp
+++ b/src/devices/backlight.cpp
@@ -145,6 +145,9 @@ double backlight::utilization(void)
 {
 	double p;
 
+	if (max_level == 0)
+		return 0.0;
+
 	p = 100.0 * (end_level + start_level) / 2 / max_level;
 	return p;
 }
-- 
1.7.5.4