interpolate_target_q assumes a sample will always be available

[mqudsi]

thread '<unnamed>' panicked at 'called `Option::unwrap()` on a `None` value', av1an-core/src/target_quality.rs:340:6

This corresponds to the last unwrap in the following code fragment:

pub fn interpolate_target_q(scores: Vec<(f64, u32)>, target: f64) -> Result<f64, Error> {
  let mut sorted = scores;
  sorted.sort_by(|a, b| a.0.partial_cmp(&b.0).unwrap());

  let keys = sorted
    .iter()
    .map(|(x, y)| Key::new(*x, f64::from(*y), Interpolation::Linear))
    .collect();

  let spline = Spline::from_vec(keys);

  Ok(spline.sample(target).unwrap())
}

The problem is that the call to spline.sample() is not guaranteed to return a value. In particular, since the requested interpolation method is Interpolation::Linear, the only way the sample could be None is if search_lower_cp() returned None before the sampling was actually attempted.

I'm not sure if it makes sense for sample_with_key() to fall back to i = 0 if search_lower_cp() returned None (for one thing, there may not be a keys[0], for another, the math might turn out wrong). It might be better to have interpolate_target_q return an Option, but its only call site is interpolated_target_q which doesn't have much room for "value not available" as it bubbles the result directly to its only caller, per_shot_target_quality() which is quite definitive and only returns an error in case of an encoder crash and not any other type of situation.

I suspect in this case scores was empty, for unknown reasons? In that case, the calculation of interpolated target might need to extend the evaluated fragment until it isn't?