wng: fix two critical bugs

the open functions of the writers in a writer node group
might be interested to know which group they belong to.
Unfortunately, the old code called open before initializing
the "wng" pointer of the writer node.

The second bug triggers for example if invalid receiver options are
given. In this case wng_close() is called with a NULL-argument as
the wng has not been opened yet. The solution is to check for NULL
pointers and return immediately in this case.