threads: fix old frames returned after avcodec_flush_buffers()

Calling avcodec_flush_buffers() and then avcodec_decode_video2() with
a 0-sized packet (to get remaining buffered frames) could incorrectly
return an old frame from before the avcodec_flush_buffers() call. Add
a loop in ff_thread_flush() to zero the got_frame field of each thread
to ensure the old frames will not be returned.

Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>
