fix null check bugs found by µmix static analyzer

src/pathops/SkPathOpsWinding.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 // given a prospective edge, compute its initial winding by projecting a ray
 // if the ray hits another edge
     // if the edge doesn't have a winding yet, hop up to that edge and start over
@@ skipping 271 matching lines @@
     int wind = 0;
     int oppWind = 0;
     for (int index = 0; index < count; ++index) {
         hit = sorted[index];
         if (!hit->fValid) {
             return false;
         }
         bool ccw = ccw_dxdy(hit->fSlope, dir);
 //        SkASSERT(!approximately_zero(hit->fT) || !hit->fValid);
         SkOpSpan* span = hit->fSpan;
-        SkOpSegment* hitSegment = span->segment();
         if (!span) {
             return false;
         }
+        SkOpSegment* hitSegment = span->segment();
         if (span->windValue() == 0 && span->oppValue() == 0) {
             continue;
         }
         if (last && SkDPoint::ApproximatelyEqual(*last, hit->fPt)) {
             return false;
         }
         if (index < count - 1) {
             const SkPoint& next = sorted[index + 1]->fPt;
             if (SkDPoint::ApproximatelyEqual(next, hit->fPt)) {
                 return false;
@@ skipping 92 matching lines @@
                 continue;
             }
             SkOpSpan* result = contour->findSortableTop(contourHead);
             if (result) {
                 return result;
             }
         } while ((contour = contour->next()));
     }
     return nullptr;
 }