engine: free after log worker thread has been stopped

[hiroyha1]

In the current code, config->log_file may be referenced in the log_push function after it is freed. This fix frees log_file after calling flb_log_destroy function so that it waits for log_worker_collector function to exit.

Fixes #8429

---

Enter [N/A] in the box, if an item is not applicable to your change.

Testing
Before we can approve your change; please submit the following in a comment:

• [N/A] Example configuration file for the change
• [N/A] Debug log output from testing the change

• Attached Valgrind output that shows no leaks or memory corruption was found
  valgrind.log

If this is a change to packaging of containers or native binaries then please confirm it works for all targets.

• [N/A] Run local packaging test showing all targets (including any new ones) build.
• [N/A] Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• [N/A] Documentation required for this feature

Backporting

• Backport to latest stable release.

---

Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

[hiroyha1]

cc: @edsiper @leonardo-albertovich Could you please review this pr?

[RepinStyle]

@edsiper and @leonardo-albertovich - any update on this review?

[RepinStyle]

@edsiper and @leonardo-albertovich - any update on this review?