bugfix for typo in skpdiff tool

Review URL: https://codereview.chromium.org/154263008

git-svn-id: http://skia.googlecode.com/svn/trunk@13408 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/tools/skpdiff/SkDiffContext.cpp b/tools/skpdiff/SkDiffContext.cpp
index a1cc665..df1c93d 100644
--- a/tools/skpdiff/SkDiffContext.cpp
+++ b/tools/skpdiff/SkDiffContext.cpp
@@ -108,7 +108,7 @@
 
         if (!differ->diff(&baselineBitmap, &testBitmap, alphaMaskPending, &diffData.fResult)) {
             // if the diff failed record -1 as the result
-            diffData.fResult = -1;
+            diffData.fResult.result = -1;
             continue;
         }