We don't grab stream_cs to change any variables checked here, except to reset
flush_event, and that cannot result in a deadlock.

The only possible deadlocks here are:

(1) between this function and EndOfStream(), which is correct, as the two
    should presumably be serialized;

(2) between this function and EndFlush(); however, in that case we expect
    BeginFlush() first, which will unblock the streaming thread.
Signed-off-by: Zebediah Figura <z.figura12@gmail.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>