Small Fix For What I Believe Is A Bug With The Auto Focus Feature

Was playing with the latest version of TinyMCE and notice I was getting the following on the console - "Uncaught TypeError: Cannot read property 'selection' of undefined". I tracked this down to the use of the auto_focus setting and I've made a correction that I believe makes sense. I've given this a basic test (i.e. running it through the code I was using) and it seems to be returning a sensible result.