The terminating '\0' is no longer included in the size of
the extradata output by the demuxer since commit
36e61e24.
E.g. if one remuxes the JACOsub sample JACOsub_capability_tester.jss
from the FATE suite, one receives a file not recognized as JACOsub
before this patch.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>