rtpenc_mpegts: Set chain->rtp_ctx only after avformat_write_header succeeded

By making sure we at each time only have one pointer set, either a
local variable or one in the context, we avoid potential double frees
in the cleanup routines. If chain->rtp_ctx is set, it is closed by
calling avformat_write_trailer, but that shouldn't be called unless
avformat_write_header succeeded.

This issue was pointed out by Andreas Cadhalpun.

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