Check the end of the tree in |ShouldInsertSpaceBetweenObjectsIfNeeded|

r927109 crrev.com/c/3185560 added a traversal to find block-
in-inline between |previous| and |next|, but there is a case
where |next| is not found by the DFS traversal.

This patch breaks the loop when it hits the end of the tree.

Bug: 1290123
Change-Id: I9d036de75bc6ae22b9c12185d940d059e781a2eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3414179


Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Auto-Submit: Koji Ishii <kojii@chromium.org>
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/main@{#963062}