Loop device remains open on error and abort

[kmbach]

If pishrink does not leave properly, the loop device remains open. Memory is blocked until the loop device is deleted. A trap handler could solve this Problem.

function cleanup() {
losetup -d "$loopback"
}

trap cleanup SIGINT SIGTERM EXIT

Please check and implement if necessary.

[framps]

@kmbach I like your proposal and modified pishrink in my fork. Please test whether it's working as expected. If yes, let me know and I will create a PR.

[kmbach]

Hi, framp, I provokad a 'ERROR: Image already shrunk to smalllest size'
And as expected - the loop drive was deleted.