fix fuzzer crash

Fail if data is
inconsistent.

R=kjlubick@google.com
BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4923

Change-Id: Ie3f65c80c0d59968b1e3fc8bc91839bedb2e3da3
Reviewed-on: https://skia-review.googlesource.com/4923
Commit-Queue: Cary Clark <caryclark@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
diff --git a/src/pathops/SkOpCoincidence.cpp b/src/pathops/SkOpCoincidence.cpp
index dadcfa5..d7ef35f 100644
--- a/src/pathops/SkOpCoincidence.cpp
+++ b/src/pathops/SkOpCoincidence.cpp
@@ -1303,6 +1303,7 @@
         }
         /* coin and opp spans may not match up. Mark the ends, and then let the interior
            get marked as many times as the spans allow */
+        FAIL_IF(!oStart->upCastable());
         start->insertCoincidence(oStart->upCast());
         end->insertCoinEnd(oEnd);
         const SkOpSegment* segment = start->segment();