crash in stacking with registration data

Siril version: both master and 1.2

Description of the bug

Crash occurs when stacking in mean/median when sequence contains shift data. Occurence of the crash is probably rare because it depends on nb of cores, available mem and height of the bands. This is due to how the offset is computed to shift images in the stack buffer.

Reproduction

Reproduction steps:

Test package attached: regcrash.zip