[PATCH] Add a check that vmdisk and disk variables are not NULL

Hello, Thank you for your comments and explanations about disk states and snapshots in general. I agree that it is quite dangerous to always delete a snapshot in case one of the variables is NULL. I modified the patch based on Michal's comment that qemuSnapshotDeleteValidate should return

[PATCH] Сheck snapshot disk is not NULL when searching it in the VM config

When creating a snapshot of a VM with multiple hard disks, the snapshot takes into account the presence of all disks in the system. If, over time, one of the disks is deleted, the snapshot will continue to store knowledge of the deleted disk. This results in the fact that at the moment of deleting