segment: Free and reinit the muxer before calling avformat_write_header

This makes sure the muxers are set up in the way they expect
with no data left around from the previous run (which could
cause various issues including memory leaks, depending on the chaine
muxer).

This fixes memory leaks with the mpegts and flv muxers. It also
makes the usage of chained muxers correct.

Signed-off-by: Martin Storsjö <martin@martin.st>
