fix sleep timeout in scheduler

In the case that the current time equals exactly the timeout of
a `sync/timeout`, the Racket process could get stuck (using no CPU)
instead of continuing as it should.

How did we not find this before? Why am I suddenly able to replicate
the problem (i.e., hitting exactly the target timeout in the secheduler
at the millisecond granularity)?

Merge to v6.1
(cherry picked from commit 76aefcb508)

racket/src/racket/src/thread.c

@@ -5170,7 +5170,7 @@ int scheme_block_until(Scheme_Ready_Fun _f, Scheme_Needs_Wakeup_Fun fdf,
       if (sleep_end) {
 	delay = (float)(sleep_end - scheme_get_inexact_milliseconds());
 	delay /= 1000.0;
-	if (delay < 0)
+	if (delay <= 0)
 	  delay = (float)0.00001;
       } else
 	delay = 0.0;