slience a false Coverity report
    
    Coverity complains that
    
       unref(nif, netcf_if);
    
    sets nif to NULL without free'ing the memory it points to. But unref()
    is a macro (defined in src/ref.h) that uses a refcount in the object
    to automatically free it when there are no more pointers to it.
    
    Since the code is correct, we add a comment that silences the Coverity warning
    
    Signed-off-by: Laine Stump <laine@laine.org>