Make sure the buffer was allocated with enough channels.
This should not happen anymore since transcode now restarts
filter chain on format changes.
Reported-by: William Blough <blough@shell.cs.fsu.edu>

fruity.avi

too fast :/

Close #9223

Close #10850

If no input or no vout, it failed.

Close #11211

read spec and throw dice

Delays the apparition of #11145