BUG/MEDIUM: h2: Don't check send_wait to know if we're in the send_list.

BUG/MEDIUM: h2: Don't check send_wait to know if we're in the send_list.

When we have to stop sending due to the stream flow control, don't check
if send_wait is NULL to know if we're in the send_list, because at this
point it'll always be NULL, while we're probably in the list.
Use LIST_ISEMPTY(&h2s->list) instead.
Failing to do so mean we might be added in the send_list when flow control
allows us to emit again, while we're already in it.
While I'm here, replace LIST_DEL + LIST_INIT by LIST_DEL_INIT.

This should be backported to 1.9.