BUG/MEDIUM: muxes: Make sure we unsubcribed when destroying mux ctx.

BUG/MEDIUM: muxes: Make sure we unsubcribed when destroying mux ctx.

In the h1 and h2 muxes, make sure we unsubscribed before destroying the
mux context.
Failing to do so will lead in a segfault later, as the connection will
attempt to dereference its conn->send_wait or conn->recv_wait, which pointed
to the now-free'd mux context.

This was introduced by commit 39a96ee16eeec51774f9f52a783cf624a0de4ccb, so
should only be backported if that commit gets backported.