Function check_simple_buffers 

fn check_simple_buffers<C: PointCoord, D: PointCoord>(
    points: &[Point<C>],
    flags: &[PointFlags],
    buffers: &DeltaBuffers<'_, D>,
) -> Result<(), ReadError>

The buffer requirements shared by both entry points to simple glyph processing.

Everything hinges on points, which carries the glyph’s real point count: each of the others is indexed or zipped by point, so one that is short either truncates the result silently or fails later with a less useful error.