MEDIUM: mux: Destroy the stream before trying to add the conn to the idle list.

MEDIUM: mux: Destroy the stream before trying to add the conn to the idle list.

In the mux_h1 and mux_h2, move the test to see if we should add the
connection in the idle list until after we destroyed the h1s/h2s, that way
later we'll be able to check if the connection has no stream at all, and if
it should be added to the server idling list.