libosmovty: properly initialize vty->fd in vty_new()

Since we're using talloc_zero(), vty->fd is initialized with 0,
which corresponds to stdin. Let's set an invalid value to prevent
potential bugs like the one fixed by the recent change [1].

[1] Icdeaea67a06da3a2f07b252e455629559ecc1829

Change-Id: Iec15649781317a23e13d2c2840a8f672050f76c1
diff --git a/src/vty/vty.c b/src/vty/vty.c
index cd252ad..8cc2a3a 100644
--- a/src/vty/vty.c
+++ b/src/vty/vty.c
@@ -128,6 +128,7 @@
 		goto out_obuf;
 
 	new->max = VTY_BUFSIZ;
+	new->fd = -1;
 
 	return new;