libavformat: Allow calling av_write_trailer with a NULL AVIOContext

Signed-off-by: Martin Storsjö <martin@martin.st>
mpegts: Support running the write_trailer function without an AVIOContext

If opening and closing dynamic buffers as AVIOContext, we may
not have any AVIOContext available when wanting to close and
deallocate the muxer. Allow calling write_trailer despite this.

Signed-off-by: Martin Storsjö <martin@martin.st>
rtpenc_mpegts: Call write_trailer for the mpegts muxer even if no output buffer exists

Since the mpegts muxer now can handle being called with a NULL
AVIOContext, we don't need to try to allocate one before calling
write_trailer.

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