fix another pathops fuzz bug

src/pathops/SkPathOpsCommon.cpp

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 #include "SkAddIntersections.h"
 #include "SkOpCoincidence.h"
 #include "SkOpEdgeBuilder.h"
 #include "SkPathOpsCommon.h"
 #include "SkPathWriter.h"
 #include "SkTSort.h"
 
 const SkOpAngle* AngleWinding(SkOpSpanBase* start, SkOpSpanBase* end, int* windingPtr,
         bool* sortablePtr) {
     // find first angle, initialize winding to computed fWindSum
     SkOpSegment* segment = start->segment();
     const SkOpAngle* angle = segment->spanToAngle(start, end);
     if (!angle) {
         *windingPtr = SK_MinS32;
         return nullptr;
     }
     bool computeWinding = false;
     const SkOpAngle* firstAngle = angle;
     bool loop = false;
     bool unorderable = false;
     int winding = SK_MinS32;
     do {
         angle = angle->next();
+        if (!angle) {
+            return nullptr;
+        }
         unorderable |= angle->unorderable();
         if ((computeWinding = unorderable || (angle == firstAngle && loop))) {
             break;    // if we get here, there's no winding, loop is unorderable
         }
         loop |= angle == firstAngle;
         segment = angle->segment();
         winding = segment->windSum(angle);
     } while (winding == SK_MinS32);
     // if the angle loop contains an unorderable span, the angle order may be useless
     // directly compute the winding in this case for each span
@@ skipping 496 matching lines @@
         if (!coincidence->apply()) {
             return false;
         }
     }
 #if DEBUG_ACTIVE_SPANS
     coincidence->debugShowCoincidence();
     DebugShowActiveSpans(contourList);
 #endif
     return true;
 }