Add null check to RemovePreviouslySelectedEmptyTableRow().

Add DCHECK(this) to node::IsDescendantOf() to get better stack trace
in ClusterFuzz when it is called on nullptr.

Bug: 1035664
Change-Id: I09d66f770eac7c231a8f6ef9b4840a826caf7971
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2019469Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: George Steel <gtsteel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735969}

third_party/blink/renderer/core/dom/node.cc

@@ -1528,6 +1528,8 @@ void Node::ClearFlatTreeNodeDataIfHostChanged(const ContainerNode& parent) {
 }
 
 bool Node::IsDescendantOf(const Node* other) const {
+  DCHECK(this);  // Necessary for clusterfuzz tooling to get a useful backtrace
+
   // Return true if other is an ancestor of this, otherwise false
   if (!other || isConnected() != other->isConnected())
     return false;

third_party/blink/renderer/core/editing/commands/delete_selection_command.cc

@@ -975,7 +975,8 @@ void DeleteSelectionCommand::RemovePreviouslySelectedEmptyTableRows(
     if (IsTableRowEmpty(end_table_row_.Get())) {
       // Don't remove end_table_row_ if it's where we're putting the ending
       // selection.
-      if (!ending_position_.AnchorNode()->IsDescendantOf(
+      if (ending_position_.IsNull() ||
+          !ending_position_.AnchorNode()->IsDescendantOf(
               end_table_row_.Get())) {
         // FIXME: We probably shouldn't remove end_table_row_ unless it's
         // fully selected, even if it is empty. We'll need to start