For certain input skpdiff segfaulted. This prevents the segfault and should fix the failed test in housekeeping.

BUG=skia:2902
R=mtklein@google.com, rmistry@google.com

Author: stephana@google.com

Review URL: https://codereview.chromium.org/546753003
diff --git a/tools/skpdiff/SkDiffContext.cpp b/tools/skpdiff/SkDiffContext.cpp
index 78d8400..4248ca3 100644
--- a/tools/skpdiff/SkDiffContext.cpp
+++ b/tools/skpdiff/SkDiffContext.cpp
@@ -289,6 +289,7 @@
         runnableDiffs[x].setup(this, baselineEntries[x], testEntries[x]);
         tg.add(&runnableDiffs[x]);
     }
+    tg.wait();
 }
 
 void SkDiffContext::outputRecords(SkWStream& stream, bool useJSONP) {