freebsd-update: restore old exit code when no updates are available locally

This unbreaks ezjail and iocell, which get into an endless loop trying to
figure out how many times "freebsd-update install" needs to be called.

PR:		229346
Submitted by:	Mike Cole <mcole36@gmail.com>
Approved by:	bapt
MFC after:	1 week

usr.sbin/freebsd-update/freebsd-update.sh

@@ -819,6 +819,7 @@ install_check_params () {
 		echo "No updates are available to install."
 		if [ $ISFETCHED -eq 0 ]; then
 			echo "Run '$0 fetch' first."
+			exit 1
 		fi
 		exit 0
 	fi