BUG/MEDIUM: mux-h2: Don't forget to quit the send list on error reports

Similar to last fix, we need to quit the send list when reporting an
error via the send side.

This should be backported to 1.9.
diff --git a/src/mux_h2.c b/src/mux_h2.c
index 20b9651..582509f 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -617,6 +617,8 @@
 		sw->events &= ~SUB_RETRY_SEND;
 		tasklet_wakeup(sw->task);
 		h2s->send_wait = NULL;
+		LIST_DEL(&h2s->list);
+		LIST_INIT(&h2s->list);
 	}
 }