fix coincident fuzzer

src/pathops/SkOpSegment.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 "SkOpCoincidence.h"
 #include "SkOpContour.h"
 #include "SkOpSegment.h"
 #include "SkPathWriter.h"
@@ skipping 344 matching lines @@
             this->debugValidate();
 #if DEBUG_ADD_T
             SkDebugf("%s alias t=%1.9g segID=%d spanID=%d\n",  __FUNCTION__, t,
                     alias->segment()->debugID(), alias->span()->debugID());
 #endif
             span->bumpSpanAdds();
             return alias;
         }
         if (t < result->fT) {
             SkOpSpan* prev = result->span()->prev();
+            if (!prev) {
+                return nullptr;
+            }
             SkOpSpan* span = insert(prev, allocator);
             span->init(this, prev, t, pt);
             this->debugValidate();
 #if DEBUG_ADD_T
             SkDebugf("%s insert t=%1.9g segID=%d spanID=%d\n", __FUNCTION__, t,
                     span->segment()->debugID(), span->debugID());
 #endif
             span->bumpSpanAdds();
             return span->ptT();
         }
@@ skipping 1396 matching lines @@
     int absOut = SkTAbs(outerWinding);
     int absIn = SkTAbs(innerWinding);
     bool result = absOut == absIn ? outerWinding < 0 : absOut < absIn;
     return result;
 }
 
 int SkOpSegment::windSum(const SkOpAngle* angle) const {
     const SkOpSpan* minSpan = angle->start()->starter(angle->end());
     return minSpan->windSum();
 }