[Autofill] Check for self-destruction after selecting a suggestions.

This UAF was introduced in crrev.com/c/3297919 by hiding the popup
in SetSelectedLine() if the window holds a pointer lock. Since hiding
the popup deletes the popup, |this| must not be accessed afterwards.

This CL is a quick fix that adds checks after calling SetSelectedLine().
The followup CLs will make this and other checks for weak_this
redundant: crrev.com/c/3317979, crrev.com/c/3318297.

Bug: 1276850, 1239496
Change-Id: I79ce4df48ae969ba13033640f2ac2ac8c322373b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3318616
Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
Auto-Submit: Christoph Schwering <schwering@google.com>
Commit-Queue: Christoph Schwering <schwering@google.com>
Cr-Commit-Position: refs/heads/main@{#949363}