It can be unset in avcodec_parameters_from_context and a value of 0
causes SIGFPE crashes.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
(cherry picked from commit 93c39db5)
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>