Fix for star slot values sifting through to the output

Since e601356, SLU will sometimes produce outputs such as `inform(from_stop="*")`, where the value has no meaning (we only know the user mentioned the slot). However, it was still interpreted as an ordinary value, leading to weird prompts such as:

    OK, from the stop *.

This fix makes HDC policy ignore all `*` values.

However, `*` still stays in dialogue tracking and changes the behavior of the policy. A permanent fix should
probably interpret it somehow.