BUG/MEDIUM: connection: don't forget to always delete the list's head

During a test it happened that a connection was deleted before the
stream it's attached to, resulting in a crash related to the fix
18a85fe ("BUG/MEDIUM: streams: Don't forget to remove the si from
the wait list.") during the LIST_DEL(). Make sure to always delete
the list's head in this case so that other elements can safely
detach later.

This is purely 1.9, no backport is needed.
diff --git a/include/proto/connection.h b/include/proto/connection.h
index 35b9dfe..dfaaad7 100644
--- a/include/proto/connection.h
+++ b/include/proto/connection.h
@@ -709,6 +709,8 @@
 /* Releases a connection previously allocated by conn_new() */
 static inline void conn_free(struct connection *conn)
 {
+	LIST_DEL(&conn->send_wait_list);
+	LIST_INIT(&conn->send_wait_list);
 	pool_free(pool_head_connection, conn);
 }