Subtraction of two unsigned months yielded a (very large) positive value.
Conversion of this to a signed value was not necessarily well defined.

Solution: do the subtraction on signed values.


mysql-test/r/events_scheduling.result:
  Add test case.
mysql-test/t/events_scheduling.test:
  Add test case.
sql/event_data_objects.cc:
  Convert month to signed before doing the subtraction.