Problem: Setting the current time zone to a negative offset and
setting the current timestamp to a very low value would trigger assert
when altering routines and views.

Solution: Make sure the query start timestamp value is correctly
converted to when altering routines and views (using my_tz_OFFSET0,
rather than the THD's current timezone).

Change-Id: I9b96adea836dd0ebd4c1582df7041c1cb150cfe9