Chez Scheme: sleep of non-positive duration now returns immediately

A negative duration would previously result in an infinite pause in
Windows.

racket/src/ChezScheme/release_notes/release_notes.stex

@@ -1961,6 +1961,12 @@ in fasl files does not generally make sense.
 %-----------------------------------------------------------------------------
 \section{Bug Fixes}\label{section:bugfixes}
 
+\subsection{\protect\scheme{sleep} with negative duration (9.5.5)}
+
+Prior to this release, \scheme{sleep} of a negative duration would
+result in an infinite pause in Windows. Now \scheme{sleep} returns
+immediately on all platforms when given a negative duration.
+
 \subsection{Buffering signals (9.5.3)}
 
 Prior to this release, only one unhandled signal was buffered for

racket/src/ChezScheme/s/7.ss

@@ -771,7 +771,9 @@
     (lambda (t)
       (unless (and (time? t) (eq? (time-type t) 'time-duration))
         ($oops who "~s is not a time record of type time-duration" t))
-      (fp (time-second t) (time-nanosecond t)))))
+      (let ([s (time-second t)])
+        (when (>= s 0)
+          (fp s (time-nanosecond t)))))))
 
 (define $scheme-greeting
   (lambda ()