Fix skpdiff segfault caused by NaN computation.

tools/skpdiff/SkPMetric.cpp

 #include <cmath>
+#include <math.h>
 
 #include "SkBitmap.h"
 #include "skpdiff_util.h"
 #include "SkPMetric.h"
 #include "SkPMetricUtil_generated.h"
 
 struct RGB {
     float r, g, b;
 };
 
@@ skipping 140 matching lines @@
         }
     }
     bitmap->unlockPixels();
     return true;
 }
 
 // From Barten SPIE 1989
 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
 // We're keeping these around for reference and in case the lookup tables are no longer desired.
 // They are no longer called by any code in this file.
 
 // From Daly 1993
  static float visual_mask(float contrast) {
     float x = powf(392.498f * contrast, 0.7f);
     x = powf(0.0153f * x, 4.0f);
@@ skipping 278 matching lines @@
     if (!bitmap_to_cielab(baseline, &baselineLAB) || !bitmap_to_cielab(test, &testLAB)) {
         return true;
     }
 
     result->poiCount = 0;
     result->result = pmetric(&baselineLAB, &testLAB, &result->poiCount);
     result->timeElapsed = get_seconds() - startTime;
 
     return true;
 }