added logic to abort applying the scPlayer() if the source element has already had an scPlayer assigned

I found through some dodgy code of my own that multiple calls of scPlayer() to a source element weren't being aborted if the scPlayer() functionality was already applied.

I think ideally all the DOM-crawling var setting logic would follow the abort code so as to lower any unnecessary overhead.

Signed-off-by: Matt Scheurich <matt@lvl99.com>