fix another pathops fuzz bug

src/pathops/SkPathOpsOp.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"
@@ skipping 18 matching lines @@
             *chase.append() = span;
    #endif
             return last->segment();
         }
         if (done) {
             continue;
         }
         int winding;
         bool sortable;
         const SkOpAngle* angle = AngleWinding(*startPtr, *endPtr, &winding, &sortable);
+        if (!angle) {
+            return nullptr;
+        }
         if (winding == SK_MinS32) {
             continue;
         }
         int sumMiWinding, sumSuWinding;
         if (sortable) {
             segment = angle->segment();
             sumMiWinding = segment->updateWindingReverse(angle);
             sumSuWinding = segment->updateOppWindingReverse(angle);
             if (segment->operand()) {
                 SkTSwap<int>(sumMiWinding, sumSuWinding);
@@ skipping 402 matching lines @@
         one.dumpHex();
         SkDebugf("two: fill=%d\n", two.getFillType());
         two.dumpHex();
         SkASSERT(0);
     }
     return true;
 #else
     return OpDebug(one, two, op, result  SkDEBUGPARAMS(false) SkDEBUGPARAMS(nullptr));
 #endif
 }