fix pathops fuzz failures

src/pathops/SkPathOpsConic.cpp

Index: src/pathops/SkPathOpsConic.cpp
diff --git a/src/pathops/SkPathOpsConic.cpp b/src/pathops/SkPathOpsConic.cpp
index 82353d67b58767b26293e6421e977e68fd302ba4..013136bd8279a67781be68f915de1d52588cb52a 100644
--- a/src/pathops/SkPathOpsConic.cpp
+++ b/src/pathops/SkPathOpsConic.cpp
@@ -48,6 +48,14 @@ SkDVector SkDConic::dxdyAtT(double t) const {
         conic_eval_tan(&fPts[0].fX, fWeight, t),
         conic_eval_tan(&fPts[0].fY, fWeight, t)
     };
+    if (result.fX == 0 && result.fY == 0) {
+        if (zero_or_one(t)) {
+            result = fPts[2] - fPts[0];
+        } else {
+            // incomplete
+            SkDebugf("!k");
+        }
+    }
     return result;
 }