Normalize adjtime() argument.

Index: ntpdate/ntpdate.c
--- ntpdate/ntpdate.c.orig
+++ ntpdate/ntpdate.c
@@ -2041,6 +2041,10 @@ l_adj_systime(
 		adjtv.tv_usec = -adjtv.tv_usec;
 	}
 
+	while (adjtv.tv_usec < 0) {
+		adjtv.tv_usec += 1000000;
+		adjtv.tv_sec -= 1;
+	}
 	if (adjtv.tv_usec != 0 && !debug) {
 		if (adjtime(&adjtv, &oadjtv) < 0) {
 			msyslog(LOG_ERR, "Can't adjust the time of day: %m");