BUG/MAJOR: mux-h2: don't wake streams after connection was destroyed

BUG/MAJOR: mux-h2: don't wake streams after connection was destroyed

In commit 477902b ("MEDIUM: connections: Get ride of the xprt_done
callback.") we added an inconditional call to h2_wake_some_streams()
in h2_wake(), though we must not do it if the connection is destroyed
or we end up with a use-after-free. In this case it's already done in
h2_process() before destroying the connection anyway.

Let's just add this test for now. A cleaner approach might consist in
doing it in the h2_process() function itself when a connection status
change is detected.

No backport is needed, this is purely 2.2.