[packaging] Add check if old install folder exists

packaging/linux/postinst

@@ -15,7 +15,8 @@ fi
 OLD_INSTALLATION_DIR="/opt/safing/portmaster"
 MIGRATED_FILE_FLAG="$OLD_INSTALLATION_DIR/migrated.txt"
 
-if [ ! -e "$MIGRATED_FILE_FLAG" ]; then
+if [ -d "$OLD_INSTALLATION_DIR" ]; then
+    if [ ! -e "$MIGRATED_FILE_FLAG" ]; then
         echo "Starting migration form v1"
 
         # Becoause the service file need to change path, first the links to the old service needs to be removed.
@@ -32,6 +33,7 @@ if [ ! -e "$MIGRATED_FILE_FLAG" ]; then
 
         touch $MIGRATED_FILE_FLAG
         echo "Migration complete"
+    fi
 fi
 
 mv /usr/bin/portmaster /usr/lib/portmaster/portmaster