BUG/MEDIUM: mux-h2: fix missing test on sending_list in previous patch

Previous commit 989539b048 ("BUG/MINOR: mux-h2: use a safe
list_for_each_entry in h2_send()") accidently lost its sending_list test,
resulting in some elements to be woken up again while already in the
sending_list and h2_unsubscribe() crashing on integrity tests (only
when built with DEBUG_DEV).

If the fix above is backported this one must be as well.
1 file changed