quartz: Return the paused timestamp from IMediaSeeking::GetCurrentPosition() if the graph is running asynchronously.

In this case "graph->stream_start" has not yet been set.

This fixes a regression with Blazblue Centralfiction, which incorrectly skipped
a video. The application called IMediaControl::Run() and
IMediaSeeking::GetCurrentPosition() in quick succession, received an incorrect
value for the latter, and consequently believed that the video had already
finished.
Signed-off-by: Zebediah Figura <z.figura12@gmail.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>