Minor bug when seeking before loading file

If you try to seek in one of the players when there is no song loaded, the console is filled with the following error message:

[quote:7fd3e]
Please report the CORE error! (33) An invalid object handle was used. getLevels …/…/src/Monitor.cpp:256
[/quote:7fd3e]

Not very important but nice to iron that error out before going final I guess.