handle crash due to kill stale lock race, fixes #5828#5829

ThomasWaldmann merged 1 commit intoborgbackup:1.1-maintborgbackup/borg:1.1-maintfrom

ThomasWaldmann:kill-stale-lock-race-1.1ThomasWaldmann/borg:kill-stale-lock-race-1.1Copy head branch name to clipboard