Make sure a device actually exists before trying to act on it. Fixes #2739

etc/rc.disable_hdd_apm

@@ -2,6 +2,9 @@
 
 ATAIDLE=/usr/local/sbin/ataidle
 for i in /dev/ad?; do
+	if [ ! -e ${i} ]; then
+		continue;
+	fi
 	SUPPORTED=`${ATAIDLE} ${i} | grep "APM Supported" | awk '{print $3;}'`
 	if [ "${SUPPORTED}" = "yes" ] ; then
 		echo Disabling APM on $i