Crash when trying to open root directory

[ozonexo3]

https://github.com/gkngkc/UnityStandaloneFileBrowser/blob/def85a6d7fe6af4ee0b4f2693a62d321ba511a14/Assets/StandaloneFileBrowser/StandaloneFileBrowserWindows.cs#L129

It crash for "C://" directory. We can add that before to fix it:

			if (System.IO.Path.GetPathRoot(directoryPath) == directoryPath)
				return directoryPath;