Bug 1093219 - m3u parsing

No need to test index at the top of the function and then again here. Just change this line to test for undefined as well as -1.  And rather than having both push and splice calls, you could just set index to this._songs.length() here so the splice call will work. 

Also, I've gotten so used to using === and !== everywhere in JS, that == and != (you have one below) look weird to me. I'd recommend just never using the two-character forms.