Scrubber errors if you scrub with no MediaElement

[s9tpepper]

If a user tries to activate the scrub thumb before the OSMFSparkPlayer has a MediaElement set to it then the component throws a null object reference error, pasted below.

TypeError: Error #1009: Cannot access a property or method of a null object reference. at ab.osmf.spark.player::OSMFSparkPlayer/_scrubVideo()[/Users/omargonzalez/Documents/work/almerblank/source/fb_i5/OSMFSparkPlayer/src/ab/osmf/spark/player/OSMFSparkPlayer.as:360] at ab.osmf.spark.player::OSMFSparkPlayer/_handleScrubbing()[/Users/omargonzalez/Documents/work/almerblank/source/fb_i5/OSMFSparkPlayer/src/ab/osmf/spark/player/OSMFSparkPlayer.as:353] at flash.events::EventDispatcher/dispatchEventFunction() at flash.events::EventDispatcher/dispatchEvent() at mx.core::UIComponent/dispatchEvent()[E:\dev\4.x\frameworks\projects\framework\src\mx\core\UIComponent.as:12528] at ab.osmf.spark.player::LoadableMediaScrubber/_dispatchScrubbing()[/Users/omargonzalez/Documents/work/almerblank/source/fb_i5/OSMFSparkPlayer/src/ab/osmf/spark/player/LoadableMediaScrubber.as:215] at ab.osmf.spark.player::LoadableMediaScrubber/_handleThumbDownMove()[/Users/omargonzalez/Documents/work/almerblank/source/fb_i5/OSMFSparkPlayer/src/ab/osmf/spark/player/LoadableMediaScrubber.as:206]