Detect overflow in timestamp[tz] subtraction.

Detect overflow in timestamp[tz] subtraction.

It's possible to overflow the int64 microseconds field of the
output interval when subtracting two timestamps.  Detect that
instead of silently returning a bogus result.

Nick Babadzhanian

Discussion: https://postgr.es/m/CABw73Uq2oJ3E+kYvvDuY04EkhhkChim2e-PaghBDjOmgUAMWGw@mail.gmail.com