daily 440.status-mailq: avoid error from dma with submit queue

dma(8) supports mailq, but not mailq -Ac to print the submission
queue.  Don't try to print that queue from the daily script if
mailq -Ac returns an error.

Reviewed by:	bapt
Differential Revision:	https://reviews.freebsd.org/D37713

usr.sbin/periodic/etc/daily/440.status-mailq

@@ -38,7 +38,7 @@ case "$daily_status_mailq_enable" in
 
 	    case "$daily_status_include_submit_mailq" in
 	    [Yy][Ee][Ss])
-		if [ -f /etc/mail/submit.cf ]
+		if [ -f /etc/mail/submit.cf ] && mailq -Ac >/dev/null 2>&1
 		then
 		    echo ""
 		    echo "Mail in submit queue:"