don't fix winding for empty paths

src/pathops/SkOpBuilder.cpp

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkMatrix.h"
 #include "SkOpEdgeBuilder.h"
 #include "SkPathPriv.h"
@@ skipping 140 matching lines @@
         reset();
         return true;
     }
     SkPath sum;
     for (int index = 0; index < count; ++index) {
         if (!Simplify(fPathRefs[index], &fPathRefs[index])) {
             reset();
             *result = original;
             return false;
         }
-        // convert the even odd result back to winding form before accumulating it
-        FixWinding(&fPathRefs[index]);
-        sum.addPath(fPathRefs[index]);
+        if (!fPathRefs[index].isEmpty()) {
+            // convert the even odd result back to winding form before accumulating it
+            FixWinding(&fPathRefs[index]);
+            sum.addPath(fPathRefs[index]);
+        }
     }
     reset();
     bool success = Simplify(sum, result);
     if (!success) {
         *result = original;
     }
     return success;
 }