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.
diff --git a/src/mux_h2.c b/src/mux_h2.c
index 6a8a762..d1fa25a 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -4160,10 +4160,8 @@
 
 	if (size <= 0) {
 		h2s->flags |= H2_SF_BLK_SFCTL;
-		if (h2s->send_wait) {
-			LIST_DEL(&h2s->list);
-			LIST_INIT(&h2s->list);
-		}
+		if (!LIST_ISEMPTY(&h2s->list))
+			LIST_DEL_INIT(&h2s->list);
 		goto end;
 	}
 
@@ -4907,10 +4905,8 @@
 
 	if (h2s->mws <= 0) {
 		h2s->flags |= H2_SF_BLK_SFCTL;
-		if (h2s->send_wait) {
-			LIST_DEL(&h2s->list);
-			LIST_INIT(&h2s->list);
-		}
+		if (!LIST_ISEMPTY(&h2s->list))
+			LIST_DEL_INIT(&h2s->list);
 		goto end;
 	}