Null check layout object when computing accname

The LayoutObject() can technically never be null here because the
`IsInSameBlockFlow` call above checks for null LayoutObjects already,
but we shouldn't be relying on that to do validation for us.

Change-Id: I598d9e33b05e13255898d7e0fc429057a099dd67
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4978019
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1215924}