Fix validation of user field in get_user_subscriptions command

I made it test for valid muc room, instead of just valid username.

This should fix issue #4197

src/mod_muc_admin.erl

@@ -564,7 +564,7 @@ get_user_rooms(User, Server) ->
       end, ejabberd_option:hosts()).
 
 get_user_subscriptions(User, Server) ->
-    User2 = validate_muc(User, <<"user">>),
+    User2 = validate_user(User, <<"user">>),
     Server2 = validate_host(Server, <<"host">>),
     Services = find_services(global),
     UserJid = jid:make(User2, Server2),
@@ -1582,6 +1582,15 @@ validate_host(Name, ArgName) ->
 	    end
     end.
 
+-spec validate_user(Name :: binary(), ArgName::binary()) -> binary().
+validate_user(Name, ArgName) ->
+    case jid:nodeprep(Name) of
+	error ->
+	    throw({error, <<"Invalid value of '",ArgName/binary,"'">>});
+	Name2 ->
+	    Name2
+    end.
+
 -spec validate_muc(Name :: binary(), ArgName::binary()) -> binary().
 validate_muc(Name, ArgName) ->
     case jid:nameprep(Name) of