org-clock-select-task: Do not offer non-printable characters in selection

* lisp/org-clock.el (org-clock-insert-selection-line): Display N/A
when non-printable character selection is requested.  Non-printable
characters cannot be entered through `read-char'.

Reported-by: Kevin Brubeck Unhammer <unhammer@fsfe.org>
Link: https://orgmode.org/list/87y2krmzol.fsf@trigram.no

lisp/org-clock.el

@@ -701,7 +701,10 @@ pointing to it."
 			org-odd-levels-only)
 		       (length prefix))))))
       (when (and cat task)
-	(insert (format "[%c] %-12s  %s\n" i cat task))
+        (if (string-match-p "[[:print:]]" (make-string 1 i))
+	    (insert (format "[%c] %-12s  %s\n" i cat task))
+          ;; Avoid non-printable characters.
+          (insert (format "[N/A] %-12s  %s\n" cat task)))
 	(cons i marker)))))
 
 (defvar org-clock-task-overrun nil