Don't update client storage usage below zero
Don't update client storage usage below zero

(cherry picked from commit 1c3b85ac92cb03bb0b88258f2771702c17b1a030)

uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend