a7359f094898 ("conf: Don't exit if sourced default route has no gateway")
was supposed to allow passt/pasta to run even if given a template interface
which has no default gateway. However a mistake in the patch means it
still requires the gateway, but doesn't require a global address for the
guest which we really do need.
This is one part (but not the only part) of the problem seen in
https://bugs.passt.top/show_bug.cgi?id=50.
Fixes: a7359f094898
Signed-off-by: David Gibson