hevc: Correctly set time_base

- Don't set a value of "1/1" when no value is present, leave the default
- Try reading the time_base information from the VPS too, not just the VUI
- Only set time_base when an SPS is activated, not when it's decoded.
- Reduce the fraction before setting it.
- Don't set anything if the fraction is invalid (because the VUI is not
  present or because the encoded value is invalid).