Bug 35418: Fix _get_usage_months

Bug 35418: Fix _get_usage_months

No longer falls in an endless loop if the last month in the report period is december.

Test plan:
1) Apply just the tests patch, run EUsage tests
prove t/db_dependent/Koha/ERM/EUsage/
2) Wait for a few seconds. Notice SushiCounter.t it hangs. CTRL+C (cancel it)
3) Apply the fix patch.
4) Run the tests again. Notice all EUsage tests pass with no issues.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Andrew Auld <andrew.auld@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>