winegstreamer/media_source: Only seek if it was requested by the caller.

winegstreamer/media_source: Only seek if it was requested by the caller.

When starting a media source, the caller can specify a timestamp to seek
to by mean of a PROPVARIANT argument. If the PROPVARIANT is empty, then
no seek operation has to take place: playback has to either restart from
the beginning (if the media source was previously in stopped state) or
from where it was left (if the media source was in paused state).

Also, when a PROPVARIANT is empty it is a bug to use its content, because
there is no guarantee that it is initialized to a sensible value.

This patch fixes both bugs by gating the seek operation with the check
that the PROPVARIANT has the right type.

Signed-off-by: Giovanni Mascellani <gmascellani@codeweavers.com>
Signed-off-by: Huw Davies <huw@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>