Only try to attach if parent device actually exists.

I used ugly "/dev/${parent}" instead of "${parentdev}", because "/dev/"
prefix for devices listed in gbde_devices variable is optional.

Reported by:	Sean McNeil <sean@mcneil.com>

etc/rc.d/gbde

@@ -82,7 +82,7 @@ gbde_start()
 		parentdev=${device%.bde}
 		parent=${parentdev#/dev/}
 		eval "lock=\${gbde_lock_${parent}-\"${gbde_lockdir}/${parent}.lock\"}"
-		if [ ! -e "${device}" ]; then
+		if [ -e "/dev/${parent}" -a ! -e "${device}" ]; then
 			echo "Configuring Disk Encryption for ${device}."
 
 			count=1