Fix nested 'unicode-bidi: isolate' can cause infinite loop

Source/core/layout/line/InlineIterator.h

Index: Source/core/layout/line/InlineIterator.h
diff --git a/Source/core/layout/line/InlineIterator.h b/Source/core/layout/line/InlineIterator.h
index dd7a299d1519c7f142525a969d1cbe6628628a41..9c74146e820e940ef8a82a7266adcb5df7f41af3 100644
--- a/Source/core/layout/line/InlineIterator.h
+++ b/Source/core/layout/line/InlineIterator.h
@@ -532,6 +532,7 @@ static inline LineLayoutItem highestContainingIsolateWithinRoot(LineLayoutItem o
             containingIsolateObj = object;
 
         object = object.parent();
+        ASSERT(object);
     }
     return containingIsolateObj;
 }