Don't return null from ReplaceSelectionCommand::insertAsListItems()

Source/core/editing/ReplaceSelectionCommand.cpp

Index: Source/core/editing/ReplaceSelectionCommand.cpp
diff --git a/Source/core/editing/ReplaceSelectionCommand.cpp b/Source/core/editing/ReplaceSelectionCommand.cpp
index 8b8f99f1294b1b0e365e04c3fd0b0dbcf2b34957..fc6340fce51e5be5bca1dec3614f1758279af162 100644
--- a/Source/core/editing/ReplaceSelectionCommand.cpp
+++ b/Source/core/editing/ReplaceSelectionCommand.cpp
@@ -1447,8 +1447,10 @@ Node* ReplaceSelectionCommand::insertAsListItems(PassRefPtr<HTMLElement> prpList
         } else
             ASSERT_NOT_REACHED();
     }
-    if (isStart || isMiddle)
-        lastNode = lastNode->previousSibling();
+    if (isStart || isMiddle) {
+        if (Node* node = lastNode->previousSibling())
+            return node;
+    }
     return lastNode;
 }