transcode: don't use video encoder video can't be sent downstream

This is the same check done for audio and spu.

Otherwise we end up sending blocks to a id->downstream_id that doesn't exist. And it crashes.

Regression from 8177a1d6.

The crash is easy to reproduce with --sout=#transcode{vcodec=FFV1,acodec=none,scodec=none}:rtp{sdp=rtsp://localhost:8554/}