fix undefined source var

[toloudis]

It is possible that the source var is undefined and then this code still checks source.src. My fix takes care of this.

Use case: I create my video element with
var $element = $("<video>").attr("src", src);
and then later call
this.player = videojs(el, {}, function() {});
after adding it to the dom.

[heff]

Thanks for this! We'll get this pulled in and released soon.

[heff]

Thank you!