Don't scroll to an invalid index path

This CL addresses a crash we see in Stable, with the following
exception:

> -[GridViewController setMode:] - Exception: Attempted to scroll the
> collection view to an out-of-bounds item (#) when there are only #
> items in section #.

This is similar to crrev/c/4946311 and crrev/c/4915697, that solved
other codepaths the same way.

Fixed: 1497330
Change-Id: I3970bf6f9aa3a3d68778dc3570e1dd6e16708038
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4992345
Auto-Submit: Louis Romero <lpromero@google.com>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Aliona Dangla <alionadangla@chromium.org>
Commit-Queue: Louis Romero <lpromero@google.com>
Cr-Commit-Position: refs/heads/main@{#1217500}