GStreamer: fix looping a media when it's loaded from qrc.

When playing a media for the second time, the backend resets the
the media by calling setMedia() with the same value.
The problem is that setMedia() clears the previous media, which is
a QFile in this case and is the one we are trying to set again...
The QFile was deleted, causing the current media to be a dangling
pointer.

Change-Id: I6854b40212fd084d1e31e756a040a02ad103b7ba
Reviewed-by: Christian Stromme <christian.stromme@digia.com>