pFilename is required parameter for save

Hello, folks!

I hope you'll find this pull request worthy. That isn't a good idea to pass invalid parameter value by default. 
There are two solutions: the first one - assign right filepath (which isn't actually good, cause we don't know wich path is right for user, it may depends on many factors, such as OS, security, etc..) , and the second, which I choosed, mark this parameter as required and check it before use. 
Any comments, taughts are very appreciated.