fix skia pathops fuzzers

src/pathops/SkOpSpan.cpp

Index: src/pathops/SkOpSpan.cpp
diff --git a/src/pathops/SkOpSpan.cpp b/src/pathops/SkOpSpan.cpp
index 26c6f2522c2d40027227e3d1d95c574614971ed9..0c47b685f0371ac944a5c26a1949bc38ad45c3e2 100755
--- a/src/pathops/SkOpSpan.cpp
+++ b/src/pathops/SkOpSpan.cpp
@@ -471,6 +471,7 @@ bool SkOpSpan::insertCoincidence(const SkOpSegment* segment, bool flipped, bool
             if (!ordered) {
                 const SkOpSpanBase* spanEnd = fNext->contains(segment)->span();
                 const SkOpPtT* start = base->ptT()->starter(spanEnd->ptT());
+                FAIL_IF(!start->span()->upCastable());
                 span = const_cast<SkOpSpan*>(start->span()->upCast());
             } else if (flipped) {
                 span = base->prev();