Gstreamer Video - Fix stop method

[jmgasper]

From the previous review:

Why is the stop method completely removed from implementation?

rtError pxVideo::stop()
{
if(mAamp)
{
mAamp->Stop();
}
return RT_OK;
}
->

rtError pxVideo::stop()
{
// if(mAamp)
// {
// mAamp->Stop();
// }
  gPlayer->release();
return RT_OK;
}