Bug: Infinite loop in org-agenda-dim-blocked-tasks

[Al Haji-Ali]

On the latest release (9.3.7), I am running into an infinite loop when clocking 
in a blocked task if `org-enforce-todo-dependencies` is set to `t`.

The (manual) call stack is:
org-agenda-clock-in
org-agenda.el:9760 org-agenda-change-all-lines
org-agenda.el:9391 org-agenda-finalize
org-agenda.el:3879 org-agenda-dim-blocked-tasks

The while loop on line 4012 is infinite in this case.
The problem is because when `org-agenda-finalize` is called, the region is 
narrowed to a single line. Then in `org-agenda-dim-blocked-tasks` the function 
`(move-beginning-of-line 2)` is called to advance the point, which does not 
work when the buffer has a single line.

In an earlier version, the command `(forward-line)` was called which moved the 
point to eol when the buffer had a single line.

-- Al

Emacs  : GNU Emacs 27.1 (build 4, x86<sub>64</sub>-pc-linux-gnu, GTK+ Version 
3.22.30, cairo version 1.15.10)
 of 2020-08-21