add fail condition to addexpanded

src/pathops/SkOpCoincidence.cpp

Index: src/pathops/SkOpCoincidence.cpp
diff --git a/src/pathops/SkOpCoincidence.cpp b/src/pathops/SkOpCoincidence.cpp
index 42ee5c6091198b4c1eab57c4a2df45d3c419b815..b72bd6cb37232fdce2534e4479e6107e177de538 100755
--- a/src/pathops/SkOpCoincidence.cpp
+++ b/src/pathops/SkOpCoincidence.cpp
@@ -136,6 +136,9 @@ bool SkOpCoincidence::addExpanded(SkChunkAlloc* allocator
             }
             if (oTest != oEnd) {
                 oTest = coin->fFlipped ? oTest->prev() : oTest->upCast()->next();
+                if (!oTest) {
+                    return false;
+                }
             }
         }
     } while ((coin = coin->fNext));