Fix undefined issues

[dvz]

Resolves #4593

[yuliu]

Sorry about this super belated reply. There are several side notes about some commits in the PR, for reference for future development.

On commit d2445ad

• Messages or UI texts may be further refined for the consistency when displaying deleted users across the popup window and the opened sent message page.

On commit b253d84

• For a thread/reply is posted by a guest, the postusername field may be empty if the guest chose not to supply a username. So, the fix may further address showing an indicative "Guest" string from a language entry.

On commit a82fdaa

• Hardcoded ID for the Guest usergroup should be avoid, really.

On commit 7ee0ec1

• As the original issue comment suggests, the guest user avatar, $post['useravatar'], may be considered.

On commit 973940e

• Although the undefined warning things is fixed for this one, the fixed variable and variables that use it are never used in those pages.

On Commit 15b90cb

• Admit it, the relevant code around this fix that counts the star images for either usergroup titles or post count titles is a mess.