The assertion fails in nmtui's ip_route_transform_from_dest_string(),
which does not initialize the address output argument to %NULL.

There are three possibilities how the API could work:

 - assert/require the user to pass in arguments which pre-initialized
   to NULL or unset.
 - always set the output arguments, even if the function fails.
 - don't bother and leave output values untouched, if function fails.

It's not clear which approach is the best. Not to bother possibliy
leaves uninitialized values, which could be error prone. Still, do
just that.

Fixes: 0b3197a3