[racketcs] udp-receive! causes invalid memory reference

[greghendershott]

On today's snapshot build for macOS.

/tmp/udp.rkt:

#lang racket/base
(require racket/udp
         racket/function)
(define port 50000)
(define u (udp-open-socket #f #f))
(udp-bind! u #f port #t)
(udp-send-to u "127.0.0.1" port #"Hello")
(define buffer (make-bytes #xffff))
(define-values (len host _port) (udp-receive! u buffer))
(displayln (list host _port (subbytes buffer 0 len)))

$ /Applications/Racket_v7.1.0.10-CS/bin/racket /tmp/udp.rkt
invalid memory reference.  Some debugging context lost
  context...:
   condition->exn
   dynamic-wind
   linklet.sls:893:4: call-with-module-prompt
   /Users/racket/build/plt/build/cs/racket/src/build/cs/compiled/expander.scm:46630:96: body of "/tmp/udp.rkt"
   /Users/racket/build/plt/build/cs/racket/src/build/cs/compiled/expander.scm:18993:13: for-loop
   /Users/racket/build/plt/build/cs/racket/src/build/cs/compiled/expander.scm:19125:5: run-module-instance!125
   /Users/racket/build/plt/build/cs/racket/src/build/cs/compiled/expander.scm:32811:5: perform-require!78
   main.sps:173:3: namespace-require+
   #%for-each
   main.sps:636:4: thunk_6

Expected result, displays:

(127.0.0.1 50000 Hello)