Prevent integer overflow within the integer-datetimes version of

Prevent integer overflow within the integer-datetimes version of
TimestampTzPlusMilliseconds.  An integer argument of more than INT_MAX/1000
milliseconds (ie, about 35 minutes) would provoke a wrong result, resulting
in incorrect enforcement of statement_timestamp values larger than that.
Bug was introduced in my rewrite of 2006-06-20, which fixed some other
overflow risks, but missed this one :-(  Per report from Elein.