forecast: Fix undefined shift if traffic selector is /0

Not an issue in typical scenarios where the plugin is used on a gateway
with roadwarriors that use /32 addresses.  But could be an issue if used
on a client that tunnels everything to the gateway.

Fixes: e5ad2e6614 ("forecast: Add the broadcast/multicast forwarding plugin called forecast")

src/libcharon/plugins/forecast/forecast_listener.c

@@ -122,7 +122,7 @@ static bool ts2in(traffic_selector_t *ts,
 	{
 		memcpy(&addr->s_addr, net->get_address(net).ptr, 4);
 		net->destroy(net);
-		mask->s_addr = htonl(0xffffffffU << (32 - bits));
+		mask->s_addr = bits ? htonl(0xffffffffU << (32 - bits)) : 0;
 		return TRUE;
 	}
 	return FALSE;