All forum containers visible to user if at node/add/forum?gids[]

On the add forum topic page, if the gids argument is supplied as 'gids[]', all forum containers are visible in the 'Forums' drop down box, regardless of whether the logged in user actually has access to see them.

The attached patch checks to see if the user is a member of the supplied group. So if 'gids[]' is the argument, no query is returned for the list of forums, since the empty string is not a valid group id.

See the patch. This is the first one I've made so let me know if I've not explained it very well here!