Local application crash via predictable temporary directory

Predictable temporary directory creation in Zim 0.42-beta1 through 0.72.1 allows local users to crash the application. At startup, Zim derives /tmp/zim-USER from the account name and calls os.chmod() on an existing path without verifying its ownership, so an attacker-owned directory makes the permission change fail. Exploitation requires another local account able to create the victim's directory first in world-writable /tmp, and the bounded consequence is failure to start Zim.