Because write_packet() fakely writes packets to muxer by queueing
them when muxer hasn't been initialized, it should also increment
frame_number fakely.
This is required because code in do_streamcopy() rely on
frame_number.

Should fix Ticket6227
Reviewed-by: James Almer <jamrial@gmail.com>
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Muhammad Faiz <mfcc64@gmail.com>