fix chrash when using /skinsdb set without parameter or /skinsdb list public

chatcommands.lua

@@ -36,11 +36,14 @@ minetest.register_chatcommand("skinsdb", {
 		end
 
 		if command == "set" then
-			local success = skins.set_player_skin(player, parameter)
-			if success then
-				return true, S("skin set to").." "..parameter
-			else
-				return false, S("invalid skin").." "..parameter
+			if parameter then
+				local success = skins.set_player_skin(player, parameter)
+				if success then
+					return true, S("skin set to").." "..parameter
+				else
+					return false, S("invalid skin").." "..parameter
+				end
+			else return false, S("Requires skin key")
 			end
 		elseif command == "list" then
 			local list

locale/skinsdb.de.tr

@@ -16,3 +16,4 @@ skin set to=Skin ist jetzt
 invalid skin=unbekannter Skin
 unknown parameter=unbekannter Parameter
 Skins=Aussehen
+Requires skin key=Benötigt Skin-Name

locale/template.txt

@@ -17,3 +17,4 @@ unknown parameter=
 unknown skin=
 Downloads the specified range of skins and shuts down the server=
 Skins=
+Requires skin key=

skin_meta_api.lua

@@ -184,6 +184,7 @@ end
 
 function skin_class:is_applicable_for_player(playername)
 	local assigned_player = self:get_meta("playername")
-	return minetest.check_player_privs(playername, {server=true}) or assigned_player == nil or assigned_player == true or
-			(assigned_player:lower() == playername:lower())
+	return assigned_player == nil or assigned_player == true or
+ 			playername and (minetest.check_player_privs(playername, {server=true}) or
+			assigned_player:lower() == playername:lower())
 end