proc_join: fix race condition

[lukileczo]

Description

Fix race condition while waiting for threads to end - lastChecked thread might've been joined by a different thread and no longer belong to ghosts list - therefore causing NULL dereference.

Motivation and Context

JIRA: RTOS-542

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

How Has This Been Tested?

• Already covered by automatic testing.
• New test added: (add PR link here).
• Tested by hand on: armv7a7-imx6ull-dataro

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing linter checks and tests passed.
• My changes generate no new compilation warnings for any of the targets.

Special treatment

• This PR needs additional PRs to work (list the PRs, preferably in merge-order).
• I will merge this PR by myself when appropriate.

[agkaminski]

LGTM