CVE-2026-48754: Incus: Nil-pointer dereference in createDependentVolumesFromBackup on disk.{Volume,VolumeSnapshots,Pool}

(*backend).createDependentVolumesFromBackup in internal/server/storage/backend.go contains a cluster of unguarded pointer derefs on every dependent-volume entry’s VolumeSnapshots[i], Volume, and Pool sub-fields. An authenticated user with can_create_instances permission on any project can crash the incusd daemon by uploading an instance backup tarball whose dependent_volumes[*] block contains a nil snapshot pointer (or omits volume: / pool:).

This is a sibling-field variant of the 2026-05-04 batch fix d768f81c0a1d985f35ae56219519822b080bf5e3 (“Properly check dependent volumes on import”). That commit added if disk == nil at the top of the outer loop, but did not guard the four sub-pointer fields the loop body dereferences naked.