Fix crash when loading comments section of pretty much every post

If this version is going to stay on github, might as well fix the bugs in it


Commit: Added some checks to make sure ‘start_link_pos’, ‘end_link_pos, and ‘lsqb’  in CommentsTableViewController were never unreal values. Prevents the app from crashing anytime a comment includes a url link, whether formatted in brackets ( […](…) ) or not.