ffmpeg: add a (possible) build race fix

ffmpeg: add a (possible) build race fix

There's been an intermittent build fail that looks like a race:
https://bugzilla.yoctoproject.org/show_bug.cgi?id=16000

While I can't say for sure if this is fixing the issue,
there's no harm in adding a backport that rearranges the
faulty code, and someone can then try to add a real fix on top
of it. Or the race goes away and we're good.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>