Fix garbage being sent at start of a UDP channel.

If meshlink_channel_send() was called before a UDP channel had finished
the handshake, it caused UTCP to send garbage data.

diff --git a/src/utcp b/src/utcp
index eb85ec4ae5f32102430e0622e3855aaba110c032..96b97ab1291e339a36952532db8ce72e7efc45a1 160000 (submodule)
--- a/src/utcp
+++ b/src/utcp
@@ -1 +1 @@
-Subproject commit eb85ec4ae5f32102430e0622e3855aaba110c032
+Subproject commit 96b97ab1291e339a36952532db8ce72e7efc45a1