[editing] Avoid null deref in CanBeAnchorNode<EditingInFlatTreeStrategy>

The PositionInFlatTree(const Node*, int) constructor checks the node
with CanBeAnchorNode(). And for EditingInFlatTreeStrategy, this uses
node->CanParticipateInFlatTree(), which may be a null deref.

Therefore, this patch changes it to check whether the node is null.

Change-Id: Icf908f586641c9b1b57ac9ae1bf4e0ea9a0f1893
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2567220
Auto-Submit: Oriol Brufau <obrufau@igalia.com>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Oriol Brufau <obrufau@igalia.com>
Cr-Commit-Position: refs/heads/master@{#832665}

third_party/blink/renderer/core/editing/position.cc

@@ -45,7 +45,7 @@ bool CanBeAnchorNode<EditingStrategy>(Node* node) {
 template <>
 bool CanBeAnchorNode<EditingInFlatTreeStrategy>(Node* node) {
   return CanBeAnchorNode<EditingStrategy>(node) &&
-         node->CanParticipateInFlatTree();
+         (!node || node->CanParticipateInFlatTree());
 }
 #endif