dav1d_flush should set frame threads' retval to 0

If dav1d_flush sets frame threads' retval to a nonzero value such as -1, the nonzero retval will be returned by the user thread as an error after commit 2a00fb6d.

@rbultje Please review. This issue was reported in Chromium issue 1302703. Thanks.

This might well fix #389 (closed) also.

approved this merge request

Patch LGTM, thanks for looking into this.

enabled an automatic merge when the pipeline for 21053698 succeeds

merged

mentioned in issue #389 (closed)

changed milestone to %1.0.0