some fuzzer fixes

should fix three or four of the PathOp asserts
triggered by the fuzzer tool.

R=kjlubick@google.com
Bug: skia:
Change-Id: I470895addf1e922da6a7c41d44d54eca92e68fb6
Reviewed-on: https://skia-review.googlesource.com/113163
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Cary Clark <caryclark@skia.org>
diff --git a/src/pathops/SkOpSpan.cpp b/src/pathops/SkOpSpan.cpp
index 28989d9..f5195ed 100644
--- a/src/pathops/SkOpSpan.cpp
+++ b/src/pathops/SkOpSpan.cpp
@@ -24,8 +24,7 @@
             return ptT;
         }
     }
-    SkASSERT(0);  // should never return deleted
-    return this;
+    return nullptr; // should never return deleted; caller must abort
 }
 
 bool SkOpPtT::contains(const SkOpPtT* check) const {
@@ -269,12 +268,6 @@
     fSpanAdds += span->fSpanAdds;
 }
 
-SkOpSpanBase* SkOpSpanBase::active() {
-    SkOpSpanBase* result = fPrev ? fPrev->next() : upCast()->next()->prev();
-    SkASSERT(this == result || fDebugDeleted);
-    return result;
-}
-
 // please keep in sync with debugCheckForCollapsedCoincidence()
 void SkOpSpanBase::checkForCollapsedCoincidence() {
     SkOpCoincidence* coins = this->globalState()->coincidence();