Bugfix: Never create invalid snapshots on backup interruption

When canceling a backup run in the wrong moment it was possible that
restic created a snapshot with an invalid "null" tree. This caused
check and other operations to fail. The backup command now properly
handles interruptions and never saves a snapshot in that case.

https://github.com/restic/restic/issues/3151
https://github.com/restic/restic/pull/3164