Prevent double allocations of string allocated config values

Prevent double allocations of string allocated config values since they can be set by default and then set again by reading the config file.

src/openrct2/config/Config.cpp

@@ -598,11 +598,13 @@ extern "C"
 
     void config_set_defaults()
     {
+        config_release();
         Config::SetDefaults();
     }
 
     bool config_open(const utf8 * path)
     {
+        config_release();
         return Config::ReadFile(path);
     }