check for bad seafile sys username

backup_new.sh

@@ -172,8 +172,12 @@ while [ $# -gt 0 ]; do
         -u|--user|--seafuser)
             # name of seafile system user
             if [ -n "$2" ]; then
+                if ! id "$2" > /dev/null 2>&1; then
+                    badParam user "$@"
+                else
                     seafUser="${2%/}"
                     shift
+                fi
             else
                 badParam empty "$@"
             fi