There is currently an ordinary check for this (which would lead to a
memleak), but given that no demuxer should ever return a packet with an
invalid stream_index it is more appropriate for this to be an assert.

FATE passes with this change.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>