Fixed crash after a world file failed to load

When a world file failed to load, a missing check was causing a null
pointer to get added to the container for loaded worlds, which would
later get referenced in WorldManager::worldForMap, causing a crash.

Problem reported by email.

src/libtiled/worldmanager.cpp

@@ -178,6 +178,8 @@ std::unique_ptr<World> WorldManager::privateLoadWorld(const QString &fileName,
 bool WorldManager::loadWorld(const QString &fileName, QString *errorString)
 {
     auto world = privateLoadWorld(fileName, errorString);
+    if (!world)
+        return false;
 
     if (mWorlds.contains(fileName))
         delete mWorlds.take(fileName);