bugfix: views: Fix index out of range in message view focus.

[ste-el-bow]

What does this PR do, and why?

Fixes error that causes zulip-term to crash at the start
When msg_btn_list is empty or focus_msg is invalid, ensure focus_msg is set to a valid index using max(0, len(msg_btn_list) - 1) instead of just len(msg_btn_list) - 1, which would cause an IndexError.

This prevents crashes when navigating to views with no messages.

External discussion & connections

• Discussed in #zulip-terminal in topic
• Fully fixes Crash at fresh start, index out of range #1589
• Partially fixes issue #
• Builds upon previous unmerged work in PR #
• Is a follow-up to work in PR #
• Requires merge of PR #
• Merge will enable work on #

How did you test this?

• Manually - Behavioral changes
• Manually - Visual changes
• Adapting existing automated tests
• Adding automated tests for new behavior (or missing tests)
• Existing automated tests should already cover this (only a refactor of tested code)

[zulipbot]

Hello @ste-el-bow, it seems like you have referenced #1589 in your pull request description, but you have not referenced them in your commit message description(s). Referencing an issue in a commit message automatically closes the corresponding issue when the commit is merged, which makes the issue tracker easier to manage.

Please run git commit --amend in your command line client to amend your commit message description with Fixes #1589..

An example of a correctly-formatted commit:

commit fabd5e450374c8dde65ec35f02140383940fe146
Author: zulipbot
Date:   Sat Mar 18 13:42:40 2017 -0700

    pull requests: Check PR commits reference when issue is referenced.

    Fixes #51.

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