This commit stops using pkt->stream_index as index in an AVFormatContext's
streams array before actually comparing the value with the count of
streams in said array. 96e5e6ab used
pkt->stream_index in prepare_input_packet() before checking and
64063512 did likewise in
write_packets_common().
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>