video{en,de}coder: Reset the codec after calling the stop() vfunc

video{en,de}coder: Reset the codec after calling the stop() vfunc

The stop() vfunc might mess with some of our fields we have just
reset, which could cause memory leaks or invalid state taken over
to later.

Also the stop() vfunc, or anything called until it from another thread,
might want to be able to use the fields that were just resetted and
become confused because of that.

In the decoder we already had a workaround for things like this happening,
this workaround is not needed anymore.