path ops: fix fuzz-found divide by zero

src/pathops/SkOpCoincidence.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.
  */
 #include "SkOpCoincidence.h"
 #include "SkOpSegment.h"
 #include "SkPathOpsTSect.h"
 
@@ skipping 73 matching lines @@
         SkOpSpanBase* start = startPtT->span();
         SkOpSpanBase* oStart = oStartPtT->span();
         const SkOpSpanBase* end = coin->fCoinPtTEnd->span();
         const SkOpSpanBase* oEnd = coin->fOppPtTEnd->span();
         SkOpSpanBase* test = start->upCast()->next();
         SkOpSpanBase* oTest = coin->fFlipped ? oStart->prev() : oStart->upCast()->next();
         while (test != end || oTest != oEnd) {
             if (!test->ptT()->contains(oTest->ptT())) {
                 // use t ranges to guess which one is missing
                 double startRange = coin->fCoinPtTEnd->fT - startPtT->fT;
+                if (!startRange) {
+                    return false;
+                }
                 double startPart = (test->t() - startPtT->fT) / startRange;
                 double oStartRange = coin->fOppPtTEnd->fT - oStartPtT->fT;
+                if (!oStartRange) {
+                    return false;
+                }
                 double oStartPart = (oTest->t() - oStartPtT->fT) / oStartRange;
                 if (startPart == oStartPart) {
                     return false;
                 }
                 SkOpPtT* newPt;
                 if (startPart < oStartPart) {
                     double newT = oStartPtT->fT + oStartRange * startPart;
                     newPt = oStart->segment()->addT(newT, SkOpSegment::kAllowAlias, allocator);
                     newPt->fPt = test->pt();
                     test->ptT()->addOpp(newPt);
@@ skipping 552 matching lines @@
     *overS = SkTMax(SkTMin(coin1s->fT, coin1e->fT), SkTMin(coin2s->fT, coin2e->fT));
     *overE = SkTMin(SkTMax(coin1s->fT, coin1e->fT), SkTMax(coin2s->fT, coin2e->fT));
     return *overS < *overE;
 }
 
 bool SkOpCoincidence::testForCoincidence(const SkCoincidentSpans* outer, const SkOpPtT* testS,
         const SkOpPtT* testE) const {
     return testS->segment()->testForCoincidence(testS, testE, testS->span(),
             testE->span(), outer->fCoinPtTStart->segment(), 120000);  // FIXME: replace with tuned
 }