Fix skpdiff segfault caused by NaN computation.

Fix skpdiff segfault caused by NaN computation.

BUG=skia:2574
Committed: https://code.google.com/p/skia/source/detail?r=14818

[mtklein, 2014-05-20 19:42:49]

lgtm

[rmistry, 2014-05-20 20:00:02]

LGTM

Thanks for working on this!

[djsollen, 2014-05-20 20:00:47]

The CQ bit was checked by djsollen@google.com

[commit-bot: I haz the power, 2014-05-20 20:01:13]

CQ is trying da patch. Follow status at
 https://skia-tree-status.appspot.com/cq/djsollen@google.com/296033004/1

[commit-bot: I haz the power, 2014-05-21 11:57:14]

Change committed as 14818

[djsollen, 2014-05-21 12:11:02]

this patch fixes the broken window builders by using our macro.

[djsollen, 2014-05-21 12:12:29]

The CQ bit was checked by djsollen@google.com

[commit-bot: I haz the power, 2014-05-21 12:13:01]

CQ is trying da patch. Follow status at
 https://skia-tree-status.appspot.com/cq/djsollen@google.com/296033004/20001

[commit-bot: I haz the power, 2014-05-21 12:13:03]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-05-21 12:13:04]

Failed to apply patch for tools/skpdiff/SkPMetric.cpp:
While running patch -p1 --forward --force --no-backup-if-mismatch;
  patching file tools/skpdiff/SkPMetric.cpp
  Hunk #1 FAILED at 159.
  1 out of 1 hunk FAILED -- saving rejects to file
tools/skpdiff/SkPMetric.cpp.rej

Patch:       tools/skpdiff/SkPMetric.cpp
Index: tools/skpdiff/SkPMetric.cpp
diff --git a/tools/skpdiff/SkPMetric.cpp b/tools/skpdiff/SkPMetric.cpp
index
beba4974b770aefaeba6b7a2280de01b7c665517..4132ea5ab0c86876c6b6e198cc827e8f59016a72
100644
--- a/tools/skpdiff/SkPMetric.cpp
+++ b/tools/skpdiff/SkPMetric.cpp
@@ -159,10 +159,12 @@ 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 (!SkScalarIsFinite(exp)  || !SkScalarIsFinite(root)) {
+        return 0;
+    }
+    return a * cyclesPerDegree * exp * root;
 }
 
 #if 0