Fix SkClipStack bug with inverse-filled difference elements

src/core/SkClipStack.cpp

Index: src/core/SkClipStack.cpp
diff --git a/src/core/SkClipStack.cpp b/src/core/SkClipStack.cpp
index e39aeeea80383e4769fd2326431bbdad5d31d5eb..06418b46ba17966a65fea7703eb2125ddc66d739 100644
--- a/src/core/SkClipStack.cpp
+++ b/src/core/SkClipStack.cpp
@@ -237,10 +237,10 @@ void SkClipStack::Element::combineBoundsDiff(FillCombo combination, const SkRect
             // is erased, so the only pixels that can remain set
             // occur w/in the intersection of the two finite bounds
             if (!fFiniteBound.intersect(prevFinite)) {
-                this->setEmpty();
-            } else {
-                fFiniteBoundType = kNormal_BoundsType;
+                fFiniteBound.setEmpty();
+                fGenID = kEmptyGenID;
             }
+            fFiniteBoundType = kNormal_BoundsType;
             break;
         case kPrev_Cur_FillCombo:
             // The most conservative result bound is that of the