NonStop: Do not call sleep() with a 0 value

This change ensures that sleep(0) is not invoked to cause unexpected
duplicate thread context switches when _REENTRANT is specified.

Fixes: #24009

Signed-off-by: Randall S. Becker <randall.becker@nexbridge.ca>

Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24012)

crypto/sleep.c

@@ -31,7 +31,8 @@ void OSSL_sleep(uint64_t millis)
     unsigned int s = (unsigned int)(millis / 1000);
     unsigned int us = (unsigned int)((millis % 1000) * 1000);
 
-    sleep(s);
+    if (s > 0)
+        sleep(s);
     usleep(us);
 # endif
 }