Normalize adjtime() argument.

Index: libntp/systime.c
--- libntp/systime.c.orig
+++ libntp/systime.c
@@ -346,6 +346,10 @@ adj_systime(
 		adjtv.tv_usec = -adjtv.tv_usec;
 		sys_residual = -sys_residual;
 	}
+	while (adjtv.tv_usec < 0) {
+		adjtv.tv_usec += 1000000;
+		adjtv.tv_sec -= 1;
+	}
 	if (adjtv.tv_sec != 0 || adjtv.tv_usec != 0) {
 		if (adjtime(&adjtv, &oadjtv) < 0) {
 			msyslog(LOG_ERR, "adj_systime: %m");