Regression since fd172185580c1ccdcfb90bbfdb59fa806fad3117;
triggered by vp4/KTkvw8dg1J8.avi in the FATE suite, but not
when running fate as this code is not used when the bitexact
flag is set.
Bisecting done by ami_stuff, patch from user Mika Fischer
in ticket #10027 (which this commit fixes).
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>