Fix skpdiff segfault caused by NaN computation.

tools/skpdiff/SkPMetric.cpp

Index: tools/skpdiff/SkPMetric.cpp
diff --git a/tools/skpdiff/SkPMetric.cpp b/tools/skpdiff/SkPMetric.cpp
index beba4974b770aefaeba6b7a2280de01b7c665517..47880ac2916d372fc1eaf46f6fa175eec8b2dc71 100644
--- a/tools/skpdiff/SkPMetric.cpp
+++ b/tools/skpdiff/SkPMetric.cpp
@@ -1,4 +1,5 @@
 #include <cmath>
+#include <math.h>
 
 #include "SkBitmap.h"
 #include "skpdiff_util.h"
@@ -159,10 +160,12 @@ static bool bitmap_to_cielab(const SkBitmap* bitmap, ImageLAB* outImageLAB) {
 static float contrast_sensitivity(float cyclesPerDegree, float luminance) {
     float a = 440.0f * powf(1.0f + 0.7f / luminance, -0.2f);
     float b = 0.3f * powf(1.0f + 100.0f / luminance, 0.15f);
-    return a *
-           cyclesPerDegree *
-           expf(-b * cyclesPerDegree) *
-           sqrtf(1.0f + 0.06f * expf(b * cyclesPerDegree));
+    float exp = expf(-b * cyclesPerDegree);
+    float root = sqrtf(1.0f + 0.06f * expf(b * cyclesPerDegree));
+    if (!std::isfinite(exp)  || !std::isfinite(root)) {
+        return 0;
+    }
+    return a * cyclesPerDegree * exp * root;
 }
 
 #if 0