The interpolateOverDefects function allows the variance to be non-positive in two ways:

• The interpolation is not constrained to be positive.
• The fallbackValue used for the variance is identical to that for the image, and may be non-positive.

If the variance can be interpolated to zero, a significant area (e.g., pixels on a coadd that have no corresponding inputs) may be detected as a source, leading to excessive memory usage when we produce heavy footprints.

This is a transfer of HSC-1186 (unresolved at time of writing).