Fix exception when editing a post

[ziegenberg]

Note: Please fill out all relevant sections and remove irrelevant ones.

🔀 Purpose of this PR:

• Fixes a bug

---

📝 Description:

• When editing a post, we get an exception with the following stack trace:

Cannot access private property mod_moodleoverflow\discussion\discussion::$userid

line 69 of /mod/moodleoverflow/classes/anonymous.php: Error thrown
line 582 of /mod/moodleoverflow/classes/post/post_control.php: call to mod_moodleoverflow\anonymous::is_post_anonymous()
line 125 of /mod/moodleoverflow/post.php: call to mod_moodleoverflow\post\post_control->build_postform() 

---

📋 Checklist

Please confirm the following (check all that apply):

• I have phpunit and/or behat tests that cover my changes or additions.
• Code passes the code checker without errors and warnings.
• Code passes the moodle-ci/cd pipeline on all supported Moodle versions or the ones the plugin supports.
• Code does not have var_dump() or var_export or any other debugging statements (or commented out code) that should not appear on the productive branch.
• I ran all tests thoroughly checking for errors. I checked if bootstrap had any changes/deprecations that require changes in the plugins UI.

[TamaroWalter]

Hi @ziegenberg, thank you for your contribution. The only problem is that the function does not work in every case with a instance of the discussion class. Because of that, I proposed another solution in #233. This PR will be closed. But I will take this bug as a motivation to continue on changing the posts and discussion structure of moodleoverflow.
Kind regards
Tamaro