[packaging] Add check if old install folder exists

packaging/linux/postinst

@@ -15,6 +15,7 @@ fi
 OLD_INSTALLATION_DIR="/opt/safing/portmaster"
 MIGRATED_FILE_FLAG="$OLD_INSTALLATION_DIR/migrated.txt"
 
+if [ -d "$OLD_INSTALLATION_DIR" ]; then
     if [ ! -e "$MIGRATED_FILE_FLAG" ]; then
         echo "Starting migration form v1"
 
@@ -33,6 +34,7 @@ if [ ! -e "$MIGRATED_FILE_FLAG" ]; then
         touch $MIGRATED_FILE_FLAG
         echo "Migration complete"
     fi
+fi
 
 mv /usr/bin/portmaster /usr/lib/portmaster/portmaster
 ln -s /usr/lib/portmaster/portmaster /usr/bin/portmaster