fix(holtWintersAberration): guard against out-of-range band index

[npazosmendez]

• Fixes a panic when the bootstrap data has fewer points than expected. This can happen if the backend storage skips non-existing points entirely instead of filing with NaNs.
• Adds a regression test reproducing the panic.
• Adds a test case for NaN handling, inspired on a graphite-web test. I compared test covarage to ensure a more faithful port, it seemed to be the only test we were missing. This passed before these changes.