luci-mod-freifunk: Fix "Undefined reference to net.iwdata"

This PR fixes an Internal Server Error when requesting the Wireless Informations in luci-mod-freifunk.
It checks an unindexed value "net.iwdata".

```
/usr/lib/lua/luci/dispatcher.lua:460: Failed to execute template dispatcher target for entry '/freifunk/status'. The called action terminated with an exception: /usr/lib/lua/luci/template.lua:97: Failed to execute template 'freifunk/public_status'.

A runtime error occured: [string "/usr/lib/lua/luci/view/freifunk/public_stat..."]:116: attempt to index field 'iwdata' (a nil value) stack traceback: [C]: in function 'assert' /usr/lib/lua/luci/dispatcher.lua:460: in function 'dispatch' /usr/lib/lua/luci/dispatcher.lua:141: in function...
```

signed-off.by: Guido Lipke <lipkegu@gmail.com>