BUG/MEDIUM: register peer sync handler in the proper order

Hervé Commowick reported a failure to resync upon restart caused by a
segfault on the old process. This is due to the data_ctx of the connection
being initialized after the stream interface.
diff --git a/src/stream_interface.c b/src/stream_interface.c
index ceed602..5f7ac3d 100644
--- a/src/stream_interface.c
+++ b/src/stream_interface.c
@@ -336,7 +336,8 @@
  * it is solicited. The task's processing function must call the applet's
  * function before returning. It must be deleted by the task handler using
  * stream_int_unregister_handler(), possibly from within the function itself.
- * It also pre-initializes applet.state to zero.
+ * It also pre-initializes applet.state to zero and the connection context
+ * to NULL.
  */
 struct task *stream_int_register_handler(struct stream_interface *si, struct si_applet *app)
 {