In the following scenario:

1. Start mergemaster
2. Interrupt it
3. < Somehow the temproot directory disappears >
4. mergemaster -r
Many bad things can happen, especially if the -i option is in use.

Therefore, add a check to make sure it still exists before we start
comparing files.

Brought to my attention by:
PR:		bin/40538
Submitted by:	Cyrille Lefevre <cyrille.lefevre@laposte.net>

usr.sbin/mergemaster/mergemaster.sh

@@ -838,6 +838,13 @@ mm_install () {
   return $?
 }
 
+if [ ! -d "${TEMPROOT}" ]; then
+	echo "*** FATAL ERROR: The temproot directory (${TEMPROOT})"
+	echo '                 has disappeared!'
+	echo ''
+	exit 1
+fi
+
 echo ''
 echo "*** Beginning comparison"
 echo ''