gossipd: fix uninitialized var.

We assert() that it's set by one of the branches (it should be!) but
if we don't hit one it's uninitialized, not NULL.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

gossipd/gossip.c

@@ -1897,7 +1897,7 @@ static void connect_failed(struct io_conn *conn, struct reaching *reach)
 
 static struct io_plan *conn_init(struct io_conn *conn, struct reaching *reach)
 {
-	struct addrinfo *ai;
+	struct addrinfo *ai = NULL;
 
 	switch (reach->addr.itype) {
 	case ADDR_INTERNAL_SOCKNAME: