connection: Document correct use of atoi()

atoi() has undefined behavior on invalid input, but here the input comes
from wayland-scanner, which is trusted.

Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>

src/connection.c

@@ -795,6 +795,7 @@ wl_message_get_since(const struct wl_message *message)
 {
 	int since;
 
+	/* This is trusted input */
 	since = atoi(message->signature);
 
 	if (since == 0)