make <menukey>-k (kill program) clear pending io

related to PR 14851

When sending a giant bytes via write-bytes, we can get into a situation
where there are lots and lots of bytes inside the port that haven't yet
been put into the repl's text% but that will, eventually, even though
the program is dead. The commit clears out any such pending IO

pkgs/drracket-pkgs/drracket/drracket/private/rep.rkt

@@ -1209,6 +1209,9 @@ TODO
       (define/public (kill-evaluation) ; =Kernel=, =Handler=
         (when user-custodian
           (custodian-shutdown-all user-custodian))
+        (clear-input-port)
+        (clear-box-input-port)
+        (clear-output-ports)
         (set! user-custodian #f))
       
       (field (eval-thread-thunks null)