Correctly handle #f in set-overlay-renderers (racket/#3753)

A `#f` value used to be converted to a list by `flatten` and passed to
`plot-area`, since `plot-area` was exported using `unsafe-provide`, it had no
further checks and crashed racket with an invalid memory reference.

See also #91

Author: alex-hhh

plot-gui-lib/plot/private/gui/snip2d.rkt

@@ -198,7 +198,7 @@
     (define the-overlay-renderers #f)
 
     (define/public (set-overlay-renderers renderers)
-      (set! the-overlay-renderers (flatten renderers))
+      (set! the-overlay-renderers (and renderers (flatten renderers)))
       (refresh))
 
     (define (draw-overlay-renderers dc x y left top right bottom)

plot-test/plot/tests/PRs/snip-overlay-renderers.rkt

@@ -44,6 +44,11 @@
          ;; the method.
          (send snip set-overlay-renderers
                (list (function cos -3 3) (list (vrule -1) (vrule 1))))
+         (send snip draw dc 0 0 0 0 10 10 0 0 'no-caret)
+
+         ;; Ensure that #f handled correctly, see
+         ;; https://github.com/racket/racket/issues/3753
+         (send snip set-overlay-renderers #f)
          (send snip draw dc 0 0 0 0 10 10 0 0 'no-caret)))
       (send tl show #f)))))