Handle integer overflow in interval justification functions.

Handle integer overflow in interval justification functions.

justify_interval, justify_hours, and justify_days didn't check for
overflow when promoting hours to days or days to months; but that's
possible when the upper field's value is already large.  Detect and
report any such overflow.

Also, we can avoid unnecessary overflow in some cases in justify_interval
by pre-justifying the days field.  (Thanks to Nathan Bossart for this
idea.)

Joe Koshakow

Discussion: https://postgr.es/m/CAAvxfHeNqsJ2xYFbPUf_8nNQUiJqkag04NW6aBQQ0dbZsxfWHA@mail.gmail.com