jets: don't bail in _cj_spot if core is atom

[dozreg-toplud]

> .*([-:dec 10.000 ~] [9 2 0 1])
dojo: hoon expression failed
> .*([-:dec 10.000 ~] -:dec)
9.999

Obviously these two expressions should produce 9.999. If you are not convinced, let's run +mink unjetted on the first expression:

> .*([-:mink [[[-:dec 10.000 ~] 9 2 0 1] _~] +>:mink] -:mink)
[0 9.999]

The difference is that the first expression goes through jet matching code since it uses a Nock 9. +dec is registered with its parent at axis 7. When _cj_spot tries to match that parent it bails with c3__exit inside of u3h macro calls, since we replaced the parent with ~. This assertion ends up breaking Nock semantics.