tunnel: Fix uninitialized struct on stack

Some `struct tunnel` sub-properties may be read whereas the structure is
not initialized, like in `ipv4.c`:

  sprintf(env_var, "VPN_ROUTE_DEST_%d", tunnel->ipv4.split_routes);

This commits `memset`s the structure to zero to avoid strange
behaviours.

Thanks to @mata-p for finding this out.

Fixes: #55

src/tunnel.c

@@ -407,6 +407,7 @@ int run_tunnel(struct vpn_config *config)
 	int ret;
 	struct tunnel tunnel;
 
+	memset(&tunnel, 0, sizeof(tunnel));
 	tunnel.config = config;
 	tunnel.on_ppp_if_up = on_ppp_if_up;
 	tunnel.on_ppp_if_down = on_ppp_if_down;