BUG/MEDIUM: stream: don't crash on out-of-memory

In case pool_alloc() fails in stream_new(), we try to detach the stream
from the list before it has been added, dereferencing a NULL. In order
to fix it, simply move the LIST_DEL call upwards.

This must be backported to 1.8.

src/stream.c

@@ -289,10 +289,10 @@ struct stream *stream_new(struct session *sess, enum obj_type *origin)
 	flt_stream_release(s, 0);
 	task_free(t);
 	tasklet_free(s->si[1].wait_event.task);
+	LIST_DEL(&s->list);
 out_fail_alloc_si1:
 	tasklet_free(s->si[0].wait_event.task);
  out_fail_alloc:
-	LIST_DEL(&s->list);
 	pool_free(pool_head_stream, s);
 	return NULL;
 }