Orphaned comments emit debug notices in dashboard and comments lists

[johnjamesjacoby]

map_meta_cap is not prepared to handle missing or invalid post ID's. To duplicate this, delete a post from the posts table without deleting its attached comments, then visit the admin dashboard with the recent comments widget visible.

The comments list table exhibits a similar issue, where the $post global is assumed to be set and valid.

This isn't something that happens naturally in WordPress core, but is something plugins are able to do directly to the database, which results in orphaned comments. The attached patches only suppress debug notices PHP spits out, and does not directly address the issue of what to do with orphaned comments, should a plugin choose to do this.

[johnjamesjacoby]

Related: Comments list table exhibits similar issue

[ocean90]

Related/Duplicate: #16956, #16651, #19590

[c3mdigital]

Duplicate of #13905.