don't fix winding for empty paths

src/pathops/SkOpSpan.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 "SkOpCoincidence.h"
 #include "SkOpContour.h"
 #include "SkOpSegment.h"
 #include "SkPathWriter.h"
@@ skipping 261 matching lines @@
 
 void SkOpSpan::detach(SkOpPtT* kept) {
     SkASSERT(!final());
     SkOpSpan* prev = this->prev();
     SkASSERT(prev);
     SkOpSpanBase* next = this->next();
     SkASSERT(next);
     prev->setNext(next);
     next->setPrev(prev);
     this->segment()->detach(this);
-    this->globalState()->coincidence()->fixUp(this->ptT(), kept);
+    SkOpCoincidence* coincidence = this->globalState()->coincidence();
+    if (coincidence) {
+        coincidence->fixUp(this->ptT(), kept);
+    }
     this->ptT()->setDeleted();
 }
 
 void SkOpSpan::init(SkOpSegment* segment, SkOpSpan* prev, double t, const SkPoint& pt) {
     SkASSERT(t != 1);
     initBase(segment, prev, t, pt);
     fCoincident = this;
     fToAngle = NULL;
     fWindSum = fOppSum = SK_MinS32;
     fWindValue = 1;
@@ skipping 15 matching lines @@
 
 void SkOpSpan::setWindSum(int windSum) {
     SkASSERT(!final());
     if (fWindSum != SK_MinS32 && fWindSum != windSum) {
         this->globalState()->setWindingFailed();
         return;
     }
     SkASSERT(!DEBUG_LIMIT_WIND_SUM || abs(windSum) <= DEBUG_LIMIT_WIND_SUM);
     fWindSum = windSum;
 }